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o
T U R
Concerning the
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AND
R V AT
ON
UPON THE
5
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O F
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ARTHOUAKE
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Particularly of
t
That great Earthquake of the firfl of November i/j^,
which proved fo fatal to the City of Lifbon, and
whofe EfFeds v^^ere felt as far as Africa, and more
or lefs throughout almofl all Europe.
By the Reverend JOHN MICH ELL, M. A
Fellow of Queen s-College, Cambridge,
«
Read at feveral Meetings of the Royal Society;
LONDON:
Printed in the Year M, D C C. L X.'
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Conjectures concernin
the Cause
9
AND
Observations upon the Ph^enomena
>
O F
A R
H Q^U A K
INT
D
C T I O N.
Read Feb. 28. 7 ArT. I .
March 6. 13.
20. 27. 1760.
has been the ge
neral
opi
niofi of philofophers, that
earthquakes owe their origin to fome
fudden explofion in the internal parts of the earth.
This
opinion IS very
agreeable to
the , phenomena.
which leem plainly to point out fomething of that
kind. The conjectures, however, concerning the
caufe of fuch an explofion, have not been yet, I think,
fufficiently fupported
fads; nor have the more
particular effeds, which will arife from it, been
traced out; and the connexion of them with the
phasnomena explained. To do this, is the intent of
the following pages ; and this we are now the better
enabled to do, as the late dreadful earthquake of the
A 2
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ift of NoveinBer 1755 fupplies us with more * fa(^'Si;
'and thofe better related, than any other earthquake
of which we have an account.
^
That thefe concuffions fliould owe their origin to
2.
fomething in thi|;,air,as it has fometimes been imagined,,
feems. very iirfe correfpond with the phsenomena.
Thifj- 1 apprehend, will fufficiently appear^ as thofe
Dlifenomena are hereafter recounted ; nor does there
apfear to be any fiich certain and regular connexion,
between earthquakes and the ftate of the air, when
they happen, as is fuppofed by thofe vvho hold this.
opinion. It is faid, for inllance, that earthquakes
always happen in calm ftill weadier : but that this is-
not always fo, may be feen in an account of the.
-f- eiarthquakes in Sicily of 165)3, "where we are told,
the "fouth winds have blown very much, which flill
have been impetuous in the mod fenfible earth-
cc
«<
cc
quakes,^ and the like has happened at other times.
3. Other examples to the fame purj)bfe we have
In an account of the earthquakes that happened in
New England in 1727 and 1728 -, the author
^t^
%
Many
^j
Tran
*' Earthquakes,
The
The
wfi
are defirous of being acquainted with this ftibjed). The author^
of it has given us, befides the aforefaid fafis, z very judicious-
abridgment of ten of the moft confiderable v^riters upon the fub-
*je£t. I have ta6:en the greateft part ofrrtyauthorities either from
tliis author, or*the Philofophicai Tranfadions, that thofe ^ Who
would 'wifli to examine them, may have an opportunity of doing
itthe more eafily J fome things only, which were not to be met
with in thefe, and whidh "yet were necefiary to my purpofe, I
have^been obliged to feek for elfewhere.
Tianf. N'^aoT. orvol.ii. p^4:08. Lo!wth
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which, fays, tliat lie could neither obferve any cbn-
between the weather and the earthquakes
nexion
nor any prognoflic of them y for that they happened
alike in all kinds of weather, at all times of the tides,
and at all times of the moon *.
4. If, however, it ihould flill be fuppofed, not-
withftanding thefe inftances to the contrary, that
there is fome general connexion between earthquakes-
fr
vet,
ihe weather, ait the time when they happen,
it is far more probable, that the air
ihould be afFedled by the caufes of earthquakes, 'than
that the earth ihould be affe(3:ed in fo extraordinary
a manner, and to fo great a depth; and that tliis.
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Tranf, N^ 409. or
vol.
vi, part ii
Earnests Abridgment. — To
opinion of Monf.
occafion, in the following manner.
*« Seneca/ tell us, that earthquakes a:
we
who exprelles
may
himfelf.
add
202.
upon
the
this
(C
Ariftotle, Pliny, and
^ ^ ^v ^'^ 4F
" ferene air. This is, indeed, often the cafe, but not always
CC
examination
firm
it. Some
adarkfky, lightenings, and fudden ftorms, as the forerunners
^^ of earthquakes."
Then relating fome inftances of fhocks that
happened in calm and ferene weather, he adds, 'f -On
li
cc
hand, it appears, from the examples, which we have before
related, that many earthquakes have happened at the time of
great rams
*fi atmofphere.
y certain prognoflic of them in the ftate o
See Memoires Hijiorlques et Fhyftques fi
Mvnf. Bertrand^ a la Hay
This
*^"
author, in thefe fefifible memoirs, has obliged the public with a
circumftantiaraccountof all thefaftshe could colled, relating ta
the earthquakes of Switzerland, or thofe of other places, th^t
feemed to be conneSed with them. The whole feems to be done
with care and '^fidelity, and without the leall attachment to any
particular f}^ilem..
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rs^nd all the other clrcumftances attending tliefe mo
tlons, ihould be owing to fome caufe refidin
the
air.
f. Let us then, rejecting this hypothefis.
that earthquakes have their origin under ground,
we need not go far in fearch of a caufe, whofe real
exiftence in nature we have certain evidence of, and
which is capable of producing all the appearances of
thefe extraordinary motions. The caufe I mean is
fubterraneous fires. Thefe fires, if a large quantity
of water fhould be let out upon them fuddenly, may
produce a vapour, whofe quantity and elaftic force
may be fully fufficient for that purpofe. The prin«
cipal fadts, from which I would prove, that thefe
iires are the real caufe of earthquakes, are as follow.
^
Section!.
- --• m
6. Firft, The fame places are fubje<a to re-
turns of earthquakes, not only at fmall inter-
vals for fome time after any confiderable one
has happened, but alfo at greater intervals of
fome ages.
7. Both thefe fadts fufficiently appear, from the
accounts we have of earthquakes. The tremblings
and fhocks of the earth at * Jamaica in 1692, at
* Sicily in 169^5, and at * Liibon in lyff, were re-
peated fometimes at larger, and fometimes at fmaller
intervals, for feveral months. The fame thing has
been obferved in all other very violent earthquakes.
At f -Lima, from the aSth Odober 1746, to the
* See the accounts of thefe in the Philof. Tranf.
f See Ant9nio d'Uiba's Voyage to Peru, paU ji. bopk i. cb. 7
a 4th
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yji^y (the time when the account oF
them was fent from thence), there had been num-
bered no lefs than 45"! Ihoeks, many of them Httle in-
ferior to the firfl great one, which deilroyed that city.
8. The returns of earthquakes alfo, in the fame
are confirmed by
P
ces
diflances of time
hiilory
Minor, have fuffered
/
"'.o'es :
Ci
&
s
and many parts of Aiia
them, in many different
ily has been fubjed: to them, as far back
the remains even of fabulous hiflory
of
Lifbon did
firil: time in
inform us
feel the effects of them for the
75S' Jamaica has frequently been
oubled with them,, fmce the Engflilli firfl fettled
ther
e
and the Spaniards, wh
wer
there befor
ufed to build their houfes of wood, and only
flory high, for fear of them
the parts adjacent.
Lima, Callao, and
were almofl totally deflroyed by
them twice, within the compafs of about fixty years
fcarce any building being left flandingi and the latter
b
both times overflowed by the fea
were
thefe the only inffcances of tlie like kind, which have
happened there-, for, from the year 15-82 to 1746,
they have had no lefs than fixteen very violent earth-
befides an infinity of lefs confiderable ones 3
firfl fettling there, were
the old inhabitants, when they faw them
and the Spaniards
told
building high houfes, that they were building their
own fepulchres -f-
5>. Secondly
* See the place above- quoted.
What, IS here faid, is taken from d'UlIoa's Voyage to Peru,
liflory and Philofophy of Earth(juakes, the Philof. Tranf. £2'<:.
' where
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p, Secondfy, Thofe places that are In the neigh-
bourhood of burning mountains, are always
fubjedt to frequent eairthquakes j and the erup-
tions of thofe mountains, when violentj are ge-
nerally attended with them.
10. Alia Minor and Conftantinople may be looked
upon as in the neighbourhood of Santerini. The
countries alfo about * ^.tna, Vefuvius, mount Haecla,
&c. afford us fufficient proofs to the fame purpofe.
-i
But, of all the places in the known world, I fuppofe
no countries are fo fubjed: to earthquakes, as -f Peru,
Chili, and all the weftern parts of South America;
nor is there any country in the known world fp full
of volcanos:
for, throughout all that long range
of mountains, known by the name of the Andes,
from 45 degrees fouth latitude, to feyeral degrees
north of the line, as alfo throughout all Mexico,
being about 5000 miles in extent, there is a con-
tinued chain of them J.
I. _
ir.
*
I
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The motion of the earth in earth
quakes is partly tremulous, and partly propa
waves, which fucceed one
at larger and fometimes at fmaller
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With.
many
Memoires ft
are mentioned above 130 repetitions of earthquakes, that have
happened, within the compafs of 960 years, in Switzerland*
* See many inftances of this in vol. ii. of Lowthorp's Abr. of
Tranf.
+ Monf.
that fcarce a week pafles without earth-
quakes in fome part of Peru, See Hift. of Earthq. p. 205.
% See the Maps of thefe countries, Condamine's
the Maranonj Acofta's Nat» Hift, of the Ifldiesj ^e
diftances
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diftances
and this latter
IS generally
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II.
propagated much farther than the former.
The former part of this propofition want
for the proof of the latter, viz.
z'
^
confirm ation :
^wave-like motion of the
:many accounts of earthquakes
able in the two, which happened at Jamaica in
the
th, we may appeal to
was very remark-
687-8 and* i6g
in an account of the former
faid, that a gentleman there faw the ground
the fea in a
wave, as the earthquake pafTed
aiong," and that he could diflinguifh the effects of it,
to fome miles diflance, hy the motion of the tops of
r b
the trees on the hills
latter, it is
H
m an account of the
'<t
faid, « the ground heaved and fwelled
like a rolling fweUing fea
fomuch, that people
could hardly Hand upon their legs by reafon of
^
3
The fame has been obferved in the earth-
quakes of t New England, where it has been very
Temarkable. A gentleman giving an account of one,
that happened there the 1 8th November 1755, fays,
the -earth rofe in a wave, which made the tops of the
trees vibrate ten feet, and that he was forced to fup-
port himfelf, to avoid falling, whilfl it was palTing.^
1 4. The fame alfo was obferved at J L^on, in
the earthquake of the ifl November 1755, as may
be
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p. 410
Phil. Tranf. N« 209. or vol. li. Lowthorp's Abridgmfint,
Tranf. vol. I. p. i, ^c
t See the accounts colleaed together, in the 49th
\
Tranf
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p. 315
faid, «' A moft dreadful earthquake ihook
£
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to
be plainly colteded from . many of ' the accounts tEatt
have been publifhed concerning it, fome of which^
alHrm it exprefly: and this wave-like motion was>
propagated to far greater diftances than the other
tremulous one, being perceived by the motion of wa^
ters, and the hanging branches in churches, . through ;
all Germany^ amongft the Alps, in Denmark, Swer
den, Norway, and all over the British ifles.
15. Fourthly, It is obferved in places, which are^
fubjed to frequent earthquakes, that they ge-
nerally come to one and the fame place, from^
the fame point of the compafs, I- may adda
alfo, that the velocity, with which they pro
ceed, (as far as one can colle(5t it from the ac-
counts of them) is the fame 5 but the velocity
of the earthquakes of different xountries is very
different.
T
\
16. Thus all. the fiiocks, that fiicceeded the firfl
great one atLiibon in 1755, as well as the firft itfelf,
came from the * north- weft. This is alTerted by
the perfon,,who fays, he was. about writing a hiflory
of the earthquakes there.: all the other accounts alio
confirm the fame thing y. for what fome fay;, that they
came from the north, and others, that they came
>f
K
1.
> '
** fhort, but quick vibrations.
foundations
« tion changed.
' perceptible paule, the nature of the mo*
every building v^as tofled like a waggon
rough Hones, vvhich l^id in ruins aim oft
A
Tfanf.
Gibraltar, fee Hift, and Philof, of Ba
P
22.
See Fhilof, Tranf. val. xlix, p
^.
from
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from the weft, cannot be looked on as any reasonable
to this, but rather the contrary. The ve-
objedion t© this,
locityairo, with which they were.all propagated, was
the fame, being at leaft equal to that of found; for
they all followed * immediately after the noife that
preceded them, or rather the noife and the earthquake
came together : and this velocity
y well
>±
the intervals between the time when the firft fhock
was felt at Liibon, and the time when it was felt at
<3ther diftant places, frord the comparifon of which.
N
feem
Ci
have travelled at the
of more
V
•f* twenty miles /?^r minute.
17. An hiflorical account of the earthquakes, which •'
have happened in J New England, fays, that, of
five conliderable ones, three are known to have come
from the fame point of the compafs, n)iz. the nortli-
weft: it is uncertain from what point the other two
came, but it is fuppofed that they .came fi-om the fame
with the former. The || velocity of thefe has been
much lefs than that of the Liibon earthquakes : this
appears from the interval between the preceding noife,
and the ihock, as well as from the wave-like motion
before- mentioned.
.*
/
See Philof. Tranf. vol. xlix- p. xr*- or Hift. and Philof. of
Earthq. p. 315.
f See Art. 97,
A-
See Philof. Tranf.
pcopa
by no
fhock
y — - _ — ^ ^
means be owing to any caufe rending
tbcy af-e
they can
for any
_ _/
locity neither greater nor lefs dian that of founds ; that is, at the
of about thirteen jniles *
B 2
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8. All tHe greater earthquakes, that Have Been'
felt at * JamaiGaj feem
the
g
o£:
them
r
have come from the fea, and,, paffing by
Port-Royal, to have gone northwards. The velocity,
of thefe alfo v^^as far fhort of the velocity of the Lif
■/
bon earthquakes.
o. The earthquake of -j^ London, on the 8th ofl
March i7fO,. was
to move
eaft to
weft.
thing happened
Ii have been credibly informed, that the fame:
a. flight fhock, which was felfc
there in the laft century, as the perfon, who told me
this, had an opportunity of obferving; for being, by
accident, in a fcalemaker's fliop at the time when it
happened,, he found that all the fcales vibrated from.
eaft to weft.
20. All the fhocks that have been lately felt at
Brigue in Valais, have hkewife come from the fame
point of the compafs, viz. the fouth J.
Fifthly y The great Liibon earthquake has been
<
11 »
fucceeded by feveral local ones fince
of which has been much lefs.
the
a 2. Such were
thofe on the borders of France and
in Barbary, ^c.
the earthquakes in Switzerland
3
thofe
209
I^owthorp's Abr. p. 4i©> ^^" , -n. -i r t- r
t See Hift. and Philof. of Earthq. p. 250. or Philof. Tranf.
i-L_p
M
The
t See Philof. Tranf. vol. xlix. p. 620.
obferved at Smyrna alfo, fee PhUof. Tranf. N° 495. o^ Martyns
Abr. vol. X. p. 526. ■ , . r.' -1 r T- r
[I See the accounts of thefe colleaed together in Pmlof. 1 rani.
vol. xlix. or in the Hift. and Philof. of Earthq.
Sect.
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Sec t. IL
23. How well foe\
thefe fads may agree with
tshe fuppofition before laid down,
fires are the caufc of earthquakes
ever, may
That fubt
doubt, how-
it is poffible
that fires fhould fubfift, which have no communica
remain ; vtz.
how
tion with the
utward air ? In anfwer to this
I
might alledge the example of green plants, which take
by fermentation, when laid together in
heap
fire -J - . -err
where the admijGTion of the outward air is fo far from
being neceffary, that it will efFedually prevent their
doing fo
t-H
But
pafs by
we have many
ilances more immediately to the purpofe. ,
24. It can hardly be fuppofed, that the fires of the
' erality of voleanos receive any fupply of frefli air
(for this muft effeaually be prevented by that vapour
which is continually rufhing out at all their vents),
and yet they fubfift, and fiequently even increafe, for.
* ' fires of the very fame
many ag
Now, thefe
kind w^ith thofe, which I fuppofe to be the caufe
©f earthquakes
the purpofe
Other fadS) ftill more exprefly
follow
5. In the earthquake of the ift of November
755, we are told, that both fmoke and light flames
and
were feen on the coaft of Portugal, near Colares
that, upon occafion of fome of the fucceeding fhocks,
a flight fmell of fulphur was perceived to accompany
foe, which came from the fea, from the fame
s.
a
C4
quarter, whence the fmoke appeared
^ 3>
^ ^
* SeePhilof. Tranf. vol.xlix. p. 414
26
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f !
ii!
Ml
I ^
4 t
'I
i if!
ill I
111
■!.i, I
► f
I 'i
'I
I
t
: ^
I ■ I
'III
:ti
1
.i:il
iif^
n
'ii'
I
I
r _
s ■
; IP
ill!
' i J
t
I If!! t
-.' I
. L .1- <
if!
Hi
ill
I t'i
!l
rl
k
V-
14
6. In an account of an earthquake in New Eng
land, it is faid, that at Newbury, forty miles from
Bofton, the earth opened, and threw up feveral cart-
loads of fand and aflies ; and that the fand was alfo
fulphur, eniitting a 'blue
(lightly impregnated
flame, when laid on burning; coals *.
27. One of th
maica
6o
has thefe words
of the earthquake in J
In Port-Royal
<C
cc
«c
and in many places all over the illand, much ful
ph
combuftible matter hath been found
1 -
cc
cc
(fuppofed to have been thrown out upon the
opening of the earth), which, upon the iirfl touch
of fire, would flame and burn like a candle.
8
cc
St. Chrifliopher
was heretofore much
cc
cc
cc
cc
troubled with earthquakes, which, upon the erup-
tion there of a great mountain of combuftible mat-
ter, which fl:ill continues, wholly ceafed, andjiave
been. felt there fince 4-
ip- Ag
v/e
told, that
the 2oth No
vember
the fea.
burning J ifland was raifed out of
Tercera
of the .Azores
which
feveral houfes were (haken down by an earth
quake
which attended the
Pt
Of it.
Th
ifland was about three leagues m diameter, -and nearly
round ; from whence it is manifdl, that the quan-
tity of pumice fl:ones and melted matter, which mufl:
have been requifite to form it, was amazingly great :
* See Philof. Tranf. N^ 409
Eames's Abr.
part ii. p. 20U
4-. See Philof. Tranf. N** 209. or vol. ii. p. 418. Lowthorp*s
Abr.
t See Philof. Tranf. N^372
Eaaie&*3 Abr,
part lie p. 203
m
t
i
-fc
1 1
>
A\
— J-
. .y^w
r -
^1^'
-i^.M^
-=- J" J
^ r ^ ■ ' 1 I
h .'
T
!
t
I
-fc
15
m all proBabillty, it muft have far exceeded all that
has been thrown out of -^tna and Vefuvius together
within the laft two thoufand years. This may ferve
to fatisfy us/ that the fire which occafioned all this,
mufl have fubfifted for many years^ not to fay ages,
and this without any communication with the exter-
nal air. It is worth obferving, that * feveral in-
ftances of this kind have happened amongft the
Azores. There are befides many marks of fubter
raneous fires about thefe iflands, feveral places fend-
ing up fmoke or flames. Thefe iflands are alfo fu^
jedl to violent and fi-equent earthquakes*.
3 pi We have more inft:ances to the fame purpofe,
near the iflknd of Santerini in the Archipelago, where
there have been feveral little iflands raifed out of the
fea by a- fubmarine volcano. The eruption of one of
thefe in the year 1708, with all the circumilances
i_-
that attended it, we have a very good account of in
the *!» Philofophical Tranfad:ionsi It was raifed in a
place where the fea had been formerly 100 fathoms
deep, and was attended with earthquakes before it
file wed itfelf above water^ as well as after. It is re-
portedi that the ifland of Santerini itfelf was origin,
nally raifed out of the fea in the fame manner ; but^
fee that as it will, we have certain accounts of new
iflands raifed' therCi or additions made to the old ones, ,
from time to time, for above i^oo years backwards^,
and there have always- been ^rthquakes. at the timcL
of thefe eruptions,.
* SceHift. and Philof. of Earthquakes, under the titles .Azores,^
Iflands raifed, ^c.
t See N** 314, 3179 and 33a. cr vol»v. part ii. p. 19^. Jones's
Abr.
ai. Anr-
L'.
f
^3
^
^
L 4
f
^
^ i
i'
II--
-■■1
■ K I
4
*N
I
\i
\
4
/■
^
X
h -
/
r
r b
m
t- ^
m
£ I
i^'-
M- '
I Y
ii
'V
. I
1 ':
;l !
! i
'i !
I 1 ^
1 I 1! ; I
;; ^'H;
I I
»i
;4 >
:;!«!:
fc \Vt
J ' I
I I hf
!''
li
id
^ ; i!
»i ;
1 . 1 ■ ,
I 'H
1 h ■
.ii
I ^ ■
' 1 'i! j '
I ( <
'. in:
..In
■ * J
! IN
^- I
»ti
I .
it
^N
':.in
''■It
m
iiiil
I
I
)
r
I
J ^
J
>
./
r
-i
^
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^ >
>f
year
[. Ariother example of the fame kind happenei
Manila, one of the Philippine iilands, in the
This alfo was attended with violent
to which that ifland, as well as the reft
IS^
thquakes
of the Philipp
very much fubje£t
3 2. We may add to thefe, the many inflances of
vaft quantities of f pumice ftones, which have beea
fometimes found floating upon the fea, at fo great a
diftance from the ihore, as well as from any known
volcano, that there can be litde doubt of their being
thrown up by fires fubfifting under the bottom of the
ocean.
with
g
33. From thefe inftances^ we may
probability, conclude, that the fires of volcanos pr
duce earthquakes
I do not, however, fuppofe, that
the
hquakes, which are frequently felt in the
the fires
neighbourhood of volcanos, are owing ^ ^
of thofe volcanos themfelves 5 for vokatios, giving
pafTage to the vapours that are there formed, fhould
rather prevent them, as in the inftance at St. Chri
34
bcfo
We alfo meet with freq
#.
nftances con-
firming the fame thing amongft the Andes. Antonio
d'Ulloa (fpeaking of what happens amongft thefe
mountains) fays, " Experience ftiews us, that, upon
" the frefh breaking out "of any volcano, it occafions
« fo violent a fhock to the earth, that all the villages,
^< which are near it, are overthrown and deftroyed.
/.
See Philof. Tranf. vol.xlix. P. 4^59
t See Philof. Tranf. N"* 372. or vol. vj. part ii, p. 204* »"«
3^*402. or vol. m part ii. p. 43- E»n»«^« ^°^»
^c
as
6
1
-)
■^
I
€C
cc
cc
th
dinth
This {hoc
of the moiintaia
*
Car
which
may, witnout
leaft impropriety, call an earthq
accompany the erupti
ke,
i
*' dom found to
after an
cc
cc
cc
<c
opening is once made ;
is perceived, it is ve
after the volcano has once found
or
y
if fome fm
fideral
t!
fo that,
TO
nt,
a
ceafe
ftandii
2
" to be on fire
The
the matter of it cont|naes
reater earthquakes, there^
S
fore, feem rather to be occafioned by otlier^ fires, th
lie deeper in the fame trad of country
erup
%
of volcanos, which happ
at the fame
with earthquakes, may
afcribed to thofe earthq
wi
more probability, be
than the earthquakes
the eruptio
whenever, at leaft, the earthq
of any confiderable extent.
fufficiently manifeft at prefent
better underfto
ply
po
/
fX
what will be faid hereafter concerning
If this don't appear
it will, perhaps, be
to the prefent pur-
local
thquakes.
/ _
* It does n^T^pear altogether certain, from the expreffion
madeufeof in the French tranflation ^^^^ "^^'^'^ lX"flt^^.
this), that Carguayrafo might not have been a V^lcan^^ "^^^
times, which it afierted to have been the cafe by ^Y^-^fTtur
mine. It is poffible alfo, that the fame "^/y^^. ^'J.?,^^ t Innv^
mentioned in the next article ; and, indeed, it .s difficult to know
it to be othervvife, in any inftance, among the Andes, where the
volcanos are generally foind at inacceffible he^^ts. But al lowmg,
that all thefe were only old volcanos, which broke out afrelh yet
thev will ferve at leaft to fwdl the number of them m the fame
neighbourhood, as well as to fhew us, that there may, very pro-
bSwv be many more, which lie hid : for thefe ihewed no marks
o t 'e'irtrfl'L, tin: by their eruption they melted a vaft quan-
tity of fnow, with which they were before covered, ^nd wh.ch
being reduced to water, did great damage, by overflowing the
country round about*
c
s
i
/
I!
♦
\ 1
t
/
i
4 ■
i>^.
*.
'\
s :.
(*
J^t
J -
ir 4
4l
1
V
4
J
«
- - 1
i i
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•I
h
,'[
:1
if
I
n
• u
I
I
}
^ 1 I
V )
nil
jflf
i
i ]
![
t
i
tl!
^1 i
m
! \
1
■ ill ;'
in
-ill;
\ I 'li .; ■
I r
.iil!;
ill
!' !
J J.
If
iijilli!
1. 1
'It
■MSN
U\ 1
L ,
'»l!!
-i ii
- I ■!
> i i.I t'> '
iM'
If"! '
■ 1
» ^ I
IK I
liil'i
I \k
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Fir
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ill' ^^
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i8
Sect. III.
35". It may be ajfked, perhap
fuppofe, that feveral fubterraneou
fi
we ihould
exift in the
g
hbourhood of
In evidence of
\
we have frequent inflances of new volcanos breakinc
out in the neighbourhood of old ones : Carguayrafo
juft mentioned, may fupply us with one example t(
this purpofei and, in the night of the 28th of Odio-
ber 1746, in which Lima and Callao were deilroyed
no lefs than four * new ones burfl forth in the ad-
jacent mountains.
^6, To the fame purpofe, we may alleg
fiances of many volcanos lying together in the
tra6t of country
cc
fo few as forty
for example, the many place:
gft the Azores, which
either do now or have formerly fent forth fmoke and
ilames j the many volcanos alfo amongft the Andes,
already mentioned : thus i^tna, Strombolo, and Ve-
too, are all in the fame
fuvius, I may add Solfatara
neighbourhood : and Monf. Condamine fays,
traced -f* lavas, exactly like thofe of Vefuvius, al
way from Florence to Naples. In :}; Iceland alfo
heh
as
the
we
h
befides Haecla
only feveral other
^
but alfo a great number of pi
y
that fend up ful
V-
* See d^UlIoa's Voyage to Peru, part ii. book L chap. 7.
t See Phil. Tranf. vol. xlix. p. 624. All thefe lavas, as well
as the volcanos juft mentioned, lie in a continued line. The fame
thing holds good in the volcanos of the Andes alfo. This is a fadl
I muft defire the reader to attend to, as it ferves to confirm a very
material do£trine, which I (hall have occafion to mention here-
after. See art. 44, 45, and 46.
t See Horrebow's Natural Hiftory gf Iceland,
phureous
-\
\
|<iar-;v
\
/
r
1
/
1.
phureous vapour
But the examples of this kind
are few inflances
duce? of" ^fm Se Volcanos, without evlde
fo frequent, that^thei
b
pr
mark
either that there have been others formerly
5
hbourhood
or that there are, at prefent, fub
•^7
^_ fires near them.
This frequency of fubterraneous, fires
the
hbourhood of
will ap|:
ilill more
neighbour J:]Ooa or voicanub, wm a^|;wc.. ^w... ...^^
probable, if we confider the internal Ih-udure ot t[
th
and, as it will be necefiary alfo
der
underftand what follows, to know a little more o
this matter, than what falls under common obferva
ive the reader fome
tion, I ihall endeavour to
count of it. . 1 r
.. The earth then (as far as one can judge from
the"* appearances), is not compofed of heaps of matter
cafually thrown together, but of regular and uniform
ftrata. Thefe ftrata, though they frequently do not
exceed a few feet, or perhaps a few inches, in thick-
sn extend in length and breadth for
\
many
nefs (
yet
oft
mile
and this without vary
the
thick
fiderably
The fame flratum alfo preferve
1
L uniform charader throughout, though the ftrata
mmediately next to each other are very often totally
/
differ
Thus, for infla
we fhall have, pe
^
hap
ftratum of potters clay j above th
flra
of coal : then another flratum of fome other kind
of clay 5 next, a
fliarp
rit fand ftone
again
again
fand ftone
again j
, next, perhaps;
above that; and it frequently hap p
of thefe exceed a few yards in thieknefs
then clav
and coal
that
Th
are, however, mai
kind of matter is
y
nftances, in which the fame
e^
A.
inded to
C 2
depth of fom(
hundred
r
r
s
r
L I
m
f
li
*:.;
L
r"
>j
&
'^ !' Ill .
• I K
M
''^t!'M ,
^ ^
t
t
l)i[
^
'L L
i; 1
■ ■ iii'i
i i W !
IS!!''
■',! ;!' '
mi
I ! 11111 1
M'j'
ii
i i !" ill
)•
- L
f '
i.:^;H'i
J I
1
El
'PI ;
P'Mli
I
U I
.N
■i
t )
•Mr' M
\
4 P
-'
--^
20
mafi
dreds of yards ; but in all thefe, a very few only
pted,
of each is not on
but
tne
is again fubdivided into a
mtinued
number
of thin laminsE, that feldom are more
three feet thick, and frequently not fo much.
39. Beiide the horizontal divifion of the earth into
thefe flrata are again divided and fhattered by
many perpendicular filH
few and narrow, but oftentimes many, and of
which are in fome pi
liderable width. The
are
alfo
■t
wh
e a particu
1
ftr
any
inltances
fliall have almoft no
fiffures at all, though the flrata both above and be
ow it are confiderably broke
th
happens fre
quently in clay, probably on account of the foftnefs
of it, which may have made it yield to the preiiure
of the fuperincumbent matter, and fill up thofe fif-
fures which it originally had ; for we fometimes meet
correfpondent
/
th inftances in mines,
wh
ere
the
fiffures in an upper and lower flratum are interrupted
in an intermediate flratum compofed of clay," or fome
fach foft matter.
40. Though thefe fiffures do fometimes correfpond
another in the upper and lower flrata, yet this
is noi generally the cafe, at leafl not to any great di
fiance: thofe clefts, however, in which the large
of metals are found
tion to this obfei
excep
through many fl
depth
for they fometimes pafs
d thofe of different kind
to
4
, From this conflitution of the earth, viz. the
of correfpondence in the fiffures of the upper
d lov/er fir
of thofe fl
hich are little or not at all fhattered, it will come to
pa
n
' ■ M*».. V^^^
-r ■'W %
.- ■ .--r^v ' '/ 't
■»ci'-v .*»*^
\
■> " .
H ^ ' ' -"
f
* .
I
\
21
pafs, tliat the earth cannot
direction * perpendicular to the hoi
eafily be feparated
in a
if we take
any confiderabie portK
horizontal an
geth
but in
the
dhefion
between one ftratum and another, it may be feparated
without diffi
4
Thofe fiifures which are at fome depth b
1
ow
the
furface of
earth, are
liy found full of
but all thofe that are below the level of
v^ater; _— - • r i,
fea, muft always be fo, either from the oozing ot th
fea, or
rather of the land waters between the ftr
43
I
e
R
of the earth are frequently very
much bent
beir
raifed in f
plac
d de
prefTed
in
ithers, and this fo
with
very
quick afcent or defc
but as thefe afcents and de
fcents
if we
great meafure, compenfate one anothe
lar
f country
ther, we may
early h
look upon the whole fet of flrata, as
rizontally. What is very remarkable, h
their fituation, is, that from moft, if no
ads of high and mountainous countries, the flii
r, m
large
th
he in a fituation more incUned to the ho
the country itfelf, the f mountainous countries being
generally,
* What I fiiid before of thofe deep clefts, in which metals are
found, will not affea this conclufion ; for tbev are confiderably
different from either perpendicular or plane feitions of earth ; they
are frequentlv interiupted by (irata of clay, or other foft matter;
and they are, in moil parts, either filled up with rubbini, or with
ores and fpars, that adhere as firmly to the rocks on both fides, as
if they com.-ofed one,c<)ntnued ftratum with them.
f It fee.T.s very probable, from many appearances, not only that
the jnountain, ous counmes are formed out of the lower flrata of the
earth, but thac fomeumes the highefl hills in them are formed
out
.-\
1^
*;
(
1
I r
I 1.
y
I L
'fc
%
^
f
f
i
r>
i
.1^
? T-
--1
r
i
J
4
4
■ n
\
\
r
L
I
p t
r^
■ I) t *
ilii
I'll ' '
m ■[
hi!!
r P ;"
r
\
\
[ i
V
i4
)l
: i t
I
■■if'i
:i
- t.
(1,.:V
■'(
'•I
1 III!!: :
tr
:l!i':'|M
Hi" i '
I ^■ I
■, ;'
I
ii
! B i
;6
■ ill''
I'.. Ill
^'l'
^ --^
\
22
lly, if
alway
formed
of the lower
ilrata of earth. This fituation of the ftrata may be
not
dy reprefented in the followin
manner
Let a number of leaves of paper, of feveral different
:s or colours, be pafled upon one another ; then
forts
bending them up
ether into a ridge in the middle
them to be reduced again to a level furface
by a plane fo paffing through
le part that had been raifed
t
raifed
off all
middle now be
and this will be a good general
1
eprefentation of moil^
mountainous countries,
if
of
together
11, large trads of
th the parts ad-
jacent, throughout the whole world
44. From this formation of the earth
it will fol-
low, 'that v/e ought to meet with the fame kinds of
earths, ftones, and minerals, appearing at the furface
g narrow flips, and
a
parallel to the greatefl
ife of any lor
ve find them
and fo
The Andes in South Amer
ridges of mountan
has been faid before, have a chain of
extend in length ab
5000
miles
faa,
as it
i^olcanos, that
hefe volcanos.
in all probability, are all derived from the f fame
out of ftrata ftill lower than the reft, which, perhaps, may always
be the cafe, where they have volcanos in them. [See a repre-
r-_*v,*: c ^v,;. ir. fK*. Platp Ficr. -2.] In other inftances, how-
ever
.V.., it often happens, that the hills, to which thefe high lands
ferve as a bafe, are not only formed cut of the ftrata next above
them, but they ftand, as it were, in a diih, as if they had depreflbd
the ground, on which they reft, by their weight. , . . ,
* Fio-. r. reprefents a feaion of a fett of ftrata, lymg m the
fituation juft defcrlbed : the feaion is fuppofed to be made at right
andes to the length of the ridge, and perpendicular to the ho-
—
rizon.
36 and 53
F
5
ftratum
i:V V',
.^^'-^v * '/'',
A**
i:\: M'm^'^
\
«
■!
I
/
- -'\^'-
>.
V
♦
/ ..
I
/
ftratum. Parallel to the Andes, is the S
her
loiw ridse of mountains, that run between the Andes
: and " thefe tv/o ridses of mountains run
<c
d the fea
within fig
r
one
anoth
&
er.
and
noft equa
>
be
" for above a thoufand leagues together *,"
each, at a medium, about twenty leagues wide,
gold and filver mines wrought by the Spaniard
found in a trad of country parallel to the diredtion of
through a great part of the length
The
, are
thefe, and
tendin
of them
45-. The fame thing is found to obtain in North
America alfo.
the river St. L
Theg
which
fe
lurence, are kept up by a long ridge of
mountains, that run nearly parallel to the eaftern coaft
In defcending from thefe towards the fea
fets -f- of flrata, and in the fame order
fame
g
^=i
rally
met with throughout the greateil part of their length
^
46. In Great Britain, we have another inflance
the fame purpofe, where the diredlion of the ridg
in
about a point from due north and fouth,
nearly from f N. by E. to S. by W. There are many
more inftances of this to be met with in the v/orld, if
we may jud
from circumftances, which make
highly probable, that it obtains
number of
and in feveral they feem to put it almoft
of doubt
47
The reader
any infl
is not to fuppofcj however, that^
the higheft rife of the ridge, and
r
* See Acofta's Natural Hiftory of the Indies.
f See Lewis Evans's Map and Account of North America.
j Of ihis I could give many undoubted proofs, if it vi^ould not
loo far exceed the limits of my prefent defign, and which, for that
reafon, I am obliged to omit. *
the
1
N
r
r
H ,
\i
^
I
. i
r
i
F.
LB
Tfcl
L
r
»"
■k
■^t
f>
^.1
I*
%
^ fc
y
1
4
r
J
. 4
i
!>iti»^"
\ Ul
1
li
4 j
Ifl
C
HH
i
3 i r^
■ 1 k' >^
/
L :
mi? i
^ it''
■r'
M 1.^
I
I ' r
l! il
■ I m
lilii
!! ill
•'III!
'I'l
II
iiW'l
I .►'
Ilt'ft
1 L
1)1
Ml
1 1 : !
I
r 'W !
^ I k
I!
- L.
t 1 '
W^^'iMl
if
fii:i.;^i If
" ■■
.:ii
^ ■ ilr I;
1 ;■ ■■!
; ■ ^f
u
\f
I I
lite,''
M
iif!!:ii'i
i
f.^i'i
.lI-i 1 I
li-yi
frf
24
of the ftrata from
^ach fide, is perfectly uniform ; for they
quently very
fiderable inequalitie
qualities are fometimes fo
for fome fmall dift
and thefe
that the flrata are
the contrary way
f r o in
This often
akes it diificult to trace the appearance I have been
the general inclination of them
relating,
which.
knowlege of the
foffil bodies of a large trad of country, it is
^ofiible to do. '
48. At confiderable diftances from large ri
mountains, the flrata,
hardly
d
of
for the moft part, alTu
early level
d
Uy formed out of the lower flrata, fo
the more" level countries are generally formed out of
thi
fel
t upper ftrata of the earth
49. Hence it comes to pafs, th
m
of
kind
fame flrata are found to extend them
way
as well
countries o
fE
jR:ance of this
id and F
n
ption of the Channel, and
breadth as in length :
the chalky and flinty
ce, which (excepting
the clays, fands.
&c. of a few counties) compofe a trad of about thr
hundred miles each way
SO-
Befides the raiiins: of the fli
m a
ridse,
flruclure of
her very remarkable app
the
though a very common
and this is what is ufually called by miners.
the
P
m
down of the ftrata : that is, the
wtiole fet
/
of llrata on one fide a cleft are funk dov/n below the
If:
ilr
of the correfpondin
ilr
on
the
fi
fome cafe
difference in the level of the
on
different fides of the cleft, fhould be
very
t
f '
y-
i
ll . «-- -
. -3-.d
***
*
I
k
'
i
very c
25
onfiderable, it miy have a great cEcd in pro
diicing fome of the fingularities of particular earth-
quakes
R
IL
3
N the former part of th
eiTay
I have re-
unted fome of the principal appearances of
rthquak
thofe particulars in the ft
quakes, as v/eu as uiuic paitiu^iax. ... — :"-~
of the earth, upon which I fuppofe the e ap-
From what has been already
th
fom
pearances to depend.
faid, I think it is fufficiently manifeft
inftances at leaft, earthquakes are adually produced
now, therefor
byfubterraneous fire,,
be ihewn, how all the appearances above^ recited
many other
minuter circumftances attending
hquakes
may be accounted for from the fame
caufe
I -
S
I
f2
The returns of earthquakes in the fame pi
either at fmall or
large
intervals of
are
y
fiftent with the caufe affigned : fubterraneous fir
from their analogy to volcanos, might reafonably be
' gh we had not
fuppofed to fubfift for many
thofe
ftances f already mentioned, which put the
X
*
Fig. 2.
(<
Ti-
the manner juil defcribed.
perpendicularlv to the horizon, and at rigl
of the cleft : an inftance of .this kind, am
at angles to the diredion
Mend
H:
amongft the coal miners of
mentioned in the Philof. Tranf. See
tthe account of it, together with a drawing, in M 360, or J
Abr. vol.iv. partii. p;. 260.
f See art. 28 to 32 '
D
«
matte;
\
\
w
i
-V I
w
i
i
i -
F
-<
[i
X
?
f.
V
^ -**
J- r-
^
J
I
*
I
h
I* 1
'1 \
V
r f
p I
I
' liiP il i
'11
k I ■t\'t -^ '
^ |ii I i ■
' h u I ■ '
1 ifi:!! U
i '-ill' I: ■
'H -I
^ IPl -kl J^ -
! ; if I
[
■''III
■M
' it
t r '
;^'
: F
" J
I I
^
■i
i
;+
/
matter out of doubt
26
And, as it frequently happens
that volcanos rage for a time, and then are quiet
again for a number of years ; fo we fee earthquakes
alfo frequently repeated for fome fmall time, and
pting, per-
now and then fome flight ihock. And this
gy between earthquakes, and the efFeds of vol
then cealing again for a long term
hap
canos, is fo gi'eat, that I think it cannot but appear
flriking to any
Both
who will read the
of
and compare them together. The raging of
volcanos is not one continued and uniform effed :
but
eifedl, that is
peated at unequal
and with unequal degrees of force : thus, for in-
ilance, we fhall have, perhaps, two or three blafls
difcharged from a volcano, fucceeding one another
^t the interval of a few feconds only : lometimes the
intervalsare of a quarter of an hour, an hour, a day, or
perhaps feveral days. And as thefe intervals are very
qual, fo is the violence of the blafts alfo
fome
times ilones, &c. are thrown, by thefe blafts, to the
diilance of fome mi
ther times
not
the diftance of a hundred yards. The fame dif-
intervals and violence of
ference
is obferved in the
the fhocks of earthquakes, wliich are repeated at
fmall intervals fon fonle time
d'
.^^'^il!
ip
111 'it
.■•\-
t
y h
■'l^r ^
I'm I
:it*Mi
II
^ ri
\
k I
. \.
^■If!
'> t
53
g
Sect. IL
frequency of earthquakes in the
ftrong
mountains, is a
ghbourhood of
gument of their proceeding from a caufe of th
burn in ^
e
fame kind : and the analogy of feveral volcanos lying
together in the fame trad: of country, as well as new
breaking out in the neighbourhood of old
I
tends
/"
>
1 l^''^'
• .<.
■ ! "i
^i
\
\
y
.'-, ' ^ /■ .^-^v
* %<
u^M^tfOL^X.
--- tj
/
I
tends greatly to confirm this opinion 3 but what makes
ftill the more probable
peculiarity in the
^\
jftrudture of the earth, ah'eady mentioned
I obferved
before, that the fame ftrata
Uy very
nd that they commonly lie more inclining fi
live, {
the mountainous
fel
than the countries them-
s : thefe circumflances make it very probable,
thofe * ftrata of combuflible materials, v/hich
break
Th
argument
* It has been imagined by fome authors, that volcanos are pro-
duced by the pyrites of veins, and that they do not owe their origin
to the matter of ftrata. In order to prove this, it is alleged, that
volcanos are generally found on the tops of mountains, and that
thofe are the places in which veins of pyrites are generally lodged.
being taken from obfervations that have their
foundation in nature, ought not to go unaftfwered. In the firfl
place, then, the pyrites of veins, or fifTures, are not found in fuf-
ficient quantities, or extending to a fufficient breadth, to be fup-
pofed capable of producing the fires of volcanos : it very rarely
happens, that we meet with a vein or fiffure five ©r fix yards wide 5
and when we meet with fuch an one, yet, perhaps, not a twentieth
part of it at moft fhall be filled with pyrites j but the fires of vol-
canos, inftead of being long and narrow, as if the matter that fup-
plied them was depofited in veins, are generally round, and of far
greater breadth than veins can be fuppofed to be.
Co
Monf. ^ .
at this time, five
or fix hundred fathoms wide ; [fee Hid. and Phi
p. 195] and the burning ifland that was raifed out of the fea near
Terce
and nearly round.
]
Befides this, it is very difficult to conceive how any matters
lodged in veins can ever take fire; for, excepting where the veins
are extremely narrow, they are almoft always drowned in a very
great quantity of water, which has free accefs to every part of
them : neither are the pyrites of veins, by any means, fo apt to
take fire of themfelves, as thofe of flrata ; and if, indeed, there are
any of them that will do fo, yet they are but few in comparifon of
thofe which will not : all thofe, which, befide iron and fulphur^
9j)ntain copper, or arfenic, even in a very fmall proportion, are not
P 2
at
I.
I
\ ■
4
n
P
I
i
.H
■--^.
r-"
1 I
'
■ - t]
iii'i
1
I,.
fi;
\i
28
break out in volcanos on the tops of the hills, are to be
found at a conliderable depth under ground in the level
and low countries near them. If this fhould be the cafe,
"\
iV
y
w ^
*
'k
! )
^
' V u
^ ^
"T r
. hh
. ii 'i i;
L J
iiiir
1 . 'I I
■ I h- 'K V
»I H I.
^ ,- 11 '
'Ml
i I J i I
Ml'
- ,: . Hi 1
h ■ I
iiiii
► J
■ i|
.1
I
■ r
■pi'
M
jiliiiiiii
-^
b I
!■■■'[!■
.1 I.
III
•iKiiti;
^ I' t ■
at all fubjeft to inflame of themfelves. On the other hand, moft of
the pyrites of ftrata, if not all of thenr, have this property more or
lefs. There are alfo two forts of ftrata, in which pyrites are
lodged in the greateft abundance, that have the fame property.
-^
/
and that frQC[uently in as great a degree as themfelves : thefe are
coals and aluminous earths, or {hale. There are fome kinds of
both thefe, that, upon being expofed to the external air for a few
months, will take fire of themfelves, and burn. Thefe two forts
of ftrata are alfo near akin to each other; they are generally found:
to accompany each others they are both of them generally inter-
mixed with, or accompanied by ftrata of iron ore; and they both
of them, for the moft part, either contain, or are lodged amongft,.
the remains of vegetable bodies; and thefe remains of vegetable
bodies, in the aluminous earths, are frequently either wholly, or
in part, converted into pyrites, or coal, or both. Numberlefs, in-
-ftances of this are to be met with in the aluminous {hale of Whitby
iand other places.
i It is very probable, that to fome ftratum of this kind the fires of
volcanos are owing ; and this feems to be confirmed by the fimi-
larity of the materi-als, which are thrown up or fublimated by the
fires of volcanos, to the matter of the aluminous earths. Solfatara
produces fulphur, alum, and fal ammoniac. The two former of thefe
are very eafily to be obtained from the aluminous earths, and, I
fuppofe,. the latter alfo; at leaft it is procurable from the foot of
common folSl coals, and probably, therefore, from the foot of that
coaly matter which is intermixed with fuch earths.
The aluminous earths, moreover, not only have feveral ftrata of
iron ore lying in them, but they alfo contain a confiderable propor-
tion of iron in their compofition. In correfpondence to this, we find
the lavas of volcanos, and other mattejrs thrown out from thence^-
frequently containing a great deal of iron, the fmall duft of them-
readily adhering to the magnet.
As to the pyrites of veins, I much doubt whether they ever con-
tain alum, or fal ammoniac; at leaft they are very rarely found to
«;ontain either the one or the other^
.VK.ir \
■ ..-^-^v
V^:.^.v^Y'-
w:*SCi.^v mim^
I
4
./
\
■ V----
Hi
k
i
^\
J
1
^
»V
- \
./.
\t
I
and if tlie fame * ftrata fliould be on fire In any places
under fuch countries, as well as on the tops of the hills,
of whatfoever kind, raifed from thefe
all
pours
fires, muft be pent
nlefs fo far as they can open
whereas the
themfelves a paffage between the ftrata ;
Dours raifed from volcanos find a vent, and are dil-
charged in blafls from the mouths of them
if, when they find fuch
they are yet capable
of iliaking the country to the diftance of ten or twenty
miles round, what may we not exped from them
■when they are confined
We may form fome id
of the force and quantity of thefe vapours from their
rfeds : it is no uncommon thing to fee them throw
-up at once, fuch clouds of fand, aflies, and pumice
\
ftones, as
^.able of darkening the whole air,
d covering the ncighbourin
miles diftance
:itry with a fho
of duft, &c. to fome
alfo.
rei
fc
of fome tons weight, are often thrown
difl
of
and Monf. B
v-^
or three miles by thefe explofions -.
,^1- tpll« nc ■ that he met with ftones
iked
:„ ;^eZ« =d ';'<;;'7ai'e Iri .hroughout .he whole extent
cf Yt? In anta to this, it may he faid, that the fame ftratum
may differ a little in the richnefs of its combuftible prmc.ples m
..J . i" _. -.- «,^-v^..c fV>^ frpniipnrv of the fiflures, either
uency
ditrerent places ; or, pciu^^po, tiiw .i^.^--^*-/ ^- " -^ ' i«-.
b the combaftlble ilratum itfelf, or the ftratum next to it, may let
n fo much water, as to prevent its takmg fire excepting m a few
daces%ut, if this once happens, the fire will not eafily be put out
acrain but i will fpread itfelf, notwithftanding the fiffures that lie in
kf way though they are filled with water j for the matter on fire
wiirbe^'in fome degree at leaft, in a fluid ftate ; and, for this reafon,.
U mu ft neXily Ixpel the ;ater from the fiffures, l^oth on ac-
count of the extenfion of its own dimenfions by the heat, and of
the weight of the fuperincumbent earth, which, preffing it, wUi
make it fpread laterally. ' :
h
fcf.
"t
^.
if
■X
i
■1 1
m^.
\
V
:a^i>
' »
-^^— ■^-
CUj^
\ \
1
V
ii
r
I
\
i
li
I rli F j!
!H 1
1 I I
>, I
■ ' IT '"
I
;■!
m
!^;i!i'i
f\
MP ' '
■'Jill
rt
1
m ;
■m '
iifesii" '
.%
.!i
'■'ill
•K
1
n III 4^1
f I F
Mtllr
ii'
f
m
■'h'
■'l!.:il ! ;
m' ' I I
f4 !
■IS''!! !
1_
3n South America
of
30
g
or nine feet diameter
had been thrown from the volcano Cotopaxi, by
one of thefe blafts, to the difla
three leag
54. If we fuppofe
of more than
/
thefc
the caufe of earthq
P
when pent
we mufl naturally
"P
expedt, from what has been jufl faid, that the moft
extenfive earthquakes fhould take their rife from the
level and low countries ; but more efpecially from the
fea. which is nothing elfe than waters covering fjch
countries.
Accordingly we find, that the great earth
quake of the ift November 175-5, which was felt
'aces near three thpufand miles diftant from each
other, took its rife from under the fea; this is mani-
feft, from that wave which accompanied it, as (hall
be fhewn hereafter. The fame thing is to be under-
flood of the earthquake that deftroyed Lima in the
year 1 744 which, it has been faid, was felt as far as
Jamaica ; and, as it was more violent than the Lifbon
rthquake, fo, if this be
muft
pro
babihty, have been more extenfive alfo. There have
been many other very extenfive earthquakes in South
America : Acoila fays, that they have been often
two, or three hun
wn to extend themfelves
dred, and fome even five hundred leagues, along the
coaft
Thefe have been generally, if not always
tended with waves from the feaj hut any minuter
T
W
* See Hift. and Philof. of Earthq. p. 195. Don Antonio d'Ulloa,
an author of great veracity, fpeaking of the fame thing, fays, tliat
" the whole plain [near Latacunga] is full of large pieces oV rocks,
^« fome of them thrown from the volcano Cotopaxi, by one of its
^« .eruptions, to the diftance.of five leagues." See his Voya<ye to
Peru, part i. book vi. chap. .1.
-circum-
y
' h
1
I
'Tf
i±h
i
I';
ti'iisif;:' ,
',P ii T
.1. ■,.■>.
,1 .. ,
\
V ..■^'
'/■ -x
■^Mi^mx'h^
i
1
■.!■.
I
\J
y
31
circumflances accompanying them are not related
Indeed it is haidly
be
pedted that they (hould
^
be obferved, much lefs that they (hould be related
when they happened in a country fo thinly inhabited,
and where one may reafonably fuppofe, that, in ge-
neral, only the gro&r and more violent efFeds would
be taken notice of.
Sect. III.
55-. I have faid before, that I imagined earth-
quakes v^ere caufed by vapours raifed from waters
fuddenly let out upon fubterraneous fires. It is not
eafy to find any other caufe capable of producing
fuch fudden and violent effeds, or of raifing fuch an
amazing quantity of vapour in fo fmall a time. That
the blalb, difcharged from volcanos, are always pro-
duced from this caufe, is highly probable 5 that they
are often fo, cannot admit of the leail doubt. There
can be no doubt, that confiderable quantities of water
muft be often let out upon the fires of thefe
■w
/
and whenever this happens
wi
be immediately
raifed
the heat of them into a vapour, whofe
elaflic force is capable of producing the mofl violenf
effeds
5:6. Both
* There are many effeds produced by the vapour of water,
when intenfely heated, which make it probable, that the force of
gunpowder is not near eq«al to it. The efFeds of an exceeding
fmall quantity of water, upon which melted metals are acci-
dentally poured, are fuch, as, I think, could in no wife be ex-
pefted from the like quantity of gunpowder. Founders, if they
are not careful, often experience thefe elFeds to their coft. An
ed
Windmill- hill, Moorfield
" The
« heafe
f
I
!*-
il
">:
n*
J
r
■^z
,'i
m.:
i
- 1
^^ ^-fc
r
^1 II (.It I
:1
^m\\
tit p. '>l
H 1
I ■ ; [
F
> i
I
! i
\iy
W
\m^-^\
I
iliiifef
J'* . '
:m^ '
► r
'■ .-.\
-n
[:m
i*-'
uai
n
i :
my \
I I
4 b
I ' -
n; ft
, ^ ll. -
lil
(
i|*:i;ii i
■";' lit' !
'Ml
1
1 H
IK
! 1
f ■■
It
\
ft'
\
r'
32
' f6. Both tlie tremulous and wave-like moti6n ob-
^rved in earthquakes, may be very well accouoted
for
4£
h'^at of the metal of the firft gun drove (o much damp into the
.. mould of the fccond, which was near it, that as foon as the
*' metal was let into it, it blew up with the greateft violence, tear-
ine up the around feme feet deep, breaking down the furnace,
untilina the houfe, killing many fpedlators on the fpot, wid. tne
ftream?of melted metal, and fcalding many others in a moft mi-
ferable manner." [See the note at the end of procefs 44th Ox the
Encxliih tranflation of Cramer's Art of affaying Metals ]
Other inftances of the violence of vapours raifed from_ water, are
frequently to be met with: one of Papin's digefters being placed
between the bars of a grate, where there was a fire, was, atter
feme time, burft by the violence of the fteam, the fire was a
blown out of the grate, and a piece of the^djgefter was dnveii
agarnft the leaf of a ftrong oak table, whxh it broke to pieces
fSee Philof. Tranf. N° 454- or Martyn's Abr. vol. vuu p. ^^^ ^
Wo
]
tells
y
lis, that h« burft a cannon by the fame means.
It has been fometimes imagined, that the vapours, which occa-
fion earthquakes, were of the fame kmd with thofe fulminating
-camps.
nftances
Sis il no th cafe ; it is .me, the force of f.ch vapours is very
leat, we have had inftances, w.,cre large beams of nmber have
Tr
136. or vol. ii
1
[f<
what is this to the force of that vapour, which could throw ftones
N
ty ton weigni lu tiic unuaucc ui wi*ww leagues,
' ' ^ that any vapour, already in the
\
form of a vapour, can, by fuddenly taking fire, mcreafe jts dimen
fions fo much, as to produce that immenfe quantity of motion,
which we' obferve in feme earthquakes ; but this is rather to be
expeded from fome folid body, fuch as water, which is capable of
bein^ converted, and that almoft inftantly, mto one of the hghteft,
and perhaps one of the moft elaftic, vapours in the world. Air,
when heated to the greateft degree that it is capable of receiving
from the hotteft fires we can make, acquires a degree^of elalticity
.,-.. _f gjj.. the vapour of
five times as ereat as that ©t common
,gu:a-
/
/
^
Jr,
33
for from fuch z
more
vaDour.
X
^
in order to trace a little
the. manner in which thefe two
motions
*"
A ^
t
V
y
mines.
r
\
gunpowder, whilft it is inflamed, has- al fo about five times the
elaftic force which it has when cold. [See Robins's excellent
traa on Gunnery.] Now, if we fuppofe a fulminating damp, of
any kind, to increafe its elafticity, when inflamed in the fame pro-
portion, this will be abundantly fufficient to make it produce an}^
effefts, which we have ever feen produced by any of the damps of
^c. And, indeed, whoever carefully examines the effe<Sls,
either of the damps of mines, or of thofe fulminating damps, that
are raifed from fome metals, when in fufion, or when they are
diflblving In acids, will rather be inclined to think, that the force
of inflamed vapours is fo far from exceeding the proportion of five
to one, that it falls confiderably (hort of it.
But though we fhould fuppofe that this proportion holds good,
where fliall we find a place capable of containing a fufficient quan-
tity of fuch a vapour, to produce the great efFeas of earthquakes ?
It will be faid, perhaps, 'in fubterraneous caverns. To this
may anfwer, that he, who is but moderately acquainted with the
ftrudure of the earth, and the materials of which it is compofed,
will be little inclined to allow of any great or extenfive caverns in
it. But, though this fljould be admitted, how ean it come to pafs
ihat thefe caverns ffiould not be filled with water ? If it is alleged,
that the water is expelled, as the vapour is formed, w^hy fhould
^ot the vapour, as it is fuppofed to be the lighter, be expelled,
rather than the water, by the fame paffages by which the water is
to be expelled ? But let us fuppofe this difficulty alfo to be got
,over, and the water to be removed, and we fhall then have a gage
for the denfity of the vapour i for it mufl be jufl fufficient to make
it capable of fuftaining a column of water, whofe height is equal
io that of the furface of the fea above the bottom of the cavern, in
/
we
fuppofed
Now
^c
vtimes the weight of water, this vapour mufl be increafed to two
and half times its original elaflicity, before it can, in any wife,
jiaife the earth above it ; and if we fuppofe it to be increafed to five
4imes its original elafticity, it will then be no more than twice abk
to do (o ; in which cafe, fo much vapour only can be difcharged
from the cavern^ to produce an earthquake, as is equal to the
E
content
t .
/
I
s
¥^7* -
r
I
rri
■11 ii>
1
\ H
iHli'i]
111
,1i!l<l.|j||
\lm\
I
I
1
> I 1
I J II., i ;
• -''I It' y I
1 1 n P' i!L, ii
ii
1 '
>F III I
' I 'Co i
^ ! i ?! i
^i'l
M
4
r;
I i
Ill-
Hi
1 Sir .!i'! i
"lift'
i \
'
i
1 ifti iJ I
■■,'.■■*'' 1
II r
^
b ^^_ ^ ^ J^ ^ bL ^
34
r
motions will be brought about, let us fuppofe the
roof over fome fubterraneous fire to fall In. If thi&
fhould be the cafe, the earth, flones, &c. of which
r
it was compofed, would immediately £nk in the
pielted matter of the fire below : hence all the water
contained in the fiiTures and cavities of the part fall-
ing in, would come in contact with the fire, and be
aimed: infiantly raifed "into vapour. From the firfb
effoit of this vapour, a cavity would be formed (be-
tween the melted matter and fuperincumbent earth)
iiiled with vapour only, before any motion would be
perceived at the iurface of the earth : this mull ne-
celfarily happen, on account of the * compreilibility
. of
r -
a
content of the cavern: and what muft the fee of that cavern
be, which could contain vapour enough to produce the earthquake
of the lit of November 17555 in which an extent of earth of nfear
three thoufand miles diameter was confiderably moved ? or how can
we fuppofe, that the roof of fuch a cavern, when fo violently
fiiaken, fliould avoid falling in? eff-"'"^^" " "' ' ' n . ,
eipeciaily,. as it is hardly to be
fuppofedj that any inflamed vapour whatfoever Ihould be able to
move the earth over thefe caverns, if they lay at any great depth^,
fmce the weight of lefs than three miles depth of earth is capable
of retaining the inPxamed vapour of gunpowder mth'in the original
dimenfions of the gunpowder itfelf ; and common air, comprefled
by the fame weight (fuppofing the known law of its compreffion to
hold fo far)^ would be of greater denfity than water.
We may afk flill farther, whence fuch vaft quantities of vapour
-fiiould be formed, or what fouices they mufl be, which would
not be exhaufted (if they were not again replenillied) by a very few
^repetitions of fuch immenfe difcharges.
* The compreilibility and elafticity of the earth, are qualities
v/hich don^t {how themfelves in any great degree in common in-
ftances, and therefore are not commonly attended to. On this
account it is, that few people are aware of the great extent of
them, or the efFecis that may arife from them, where exceeding
large quantities of matter are concerned, and where the comprel-
■ iiv
^— H^
^
I
\
'V.v,,
tli';;iii|l 1
rhlL
^hm \
L '
- / -■-/'
^^JKJl
1
T.
it
^
I
I
\
of all
35
kinds of earth, ilones, &c.
n of the materials imm
4
but as the com
over
the
would
five
force is Immenfely great. The comprcfiibnity and elaRIcitv
of the earth may be collected, in fome nieafure, from the vibration
o^ the walls of houfes, occafioned by the paffing of carriages in the
ilreets next to them. Another inftance to the fame purpofe, may
be taken from the vibrations of iteeples, occafioned by the ringing
of bells, or by gufts of vi^ind : not only fpires are moved very con-
fiderably by this^means, but even ftrong towers will, fometimes, be
made to vibrate feveral inches, without any disjointing of the mor-
tar, or rubbing of the ftones againft one another. Nov/, it is ma-
nlfeft, that this could not happen, without a confiderable degree of
compreffibility and elafticity in the materials, of which they are
compofed : and if fuch fmall things as the weight of fteeples, and
the motion of bells in them, or a guft of wind, are capable of pro-
ducing fuch efFecfts, what may we not expe£l fi-om the weight of
great depths of earth ? There are fome circumftances, which feem
to make it not altogether improbable, that the form and internal
firudure of the earth depend, in a great meafure, upon the com-
preffibility and elaflicity of it. There are feveral things that feeni
to argue a confiderably greater denfity in the internal, than the ex-
ternal part of the earth ; and why may not this greater denfity be
owinff to the compreffion of the internal parts arlfing from the
weight of the fuperincumbent matter, fmce it is probable, that
■the matter, of which the earth is compofed, is pretty much
of the fame kind
throughout ?
There is a
gument for the earth's owing its form, in fome meafur
ftronger ar-
to
the fame caufe j
u<
accounts. of the meafures of a degree of the meridian in France,
Sweden, and America] at the equator, than at the poles, ^ in a
.greater proportion than it would be on account of the centrifugal
force, if it v/as of uniform denfity; buty if we fuppofe the earth
to be of lefs denfity in,3n seguatorial diameter than in the axis, th
.whole will then be eafily accounted for, from the rifing of the earth
a little by its elaflicity, the v/e:ght being in part taken off by the
diurnal rotation : and that the earth is really a little denfer in the
axis, than in the sequatorial diameter, feems highly probable, from
;the experiments of pendulums compared with aftronomical obferva-
jtiojis;
ived
E 2
reconciled
\
, ^
H
^
■^y
-f^'
u
I
r-
t-
f i
i
ii;
tf
m
rl-'
-i
-i- -»^-
li
nil
■ BFT 'ki
iV.
1
\
\
i
I ■ Jl ^^ :'h !
L
ll
I I
Hi
p^hl
^^
J ii;r 'in f' -
ll
Hi
(■
I
m i
it
■ \
t
^^teiiil
r ' ■
■ iWi .
Ml 't .'liM '
H M r L fi »
:'ll!flii|.'i' ■
1 h
ll'
"I'
[■-">„,
I -
w
r ,
' r
■■■*i» !': !
Ir'
V-'l^
PI I
,. i ■■■'■!( !
L , I'M], f
%:^
-■'■■ ^liil i
. ■:'.■■*"■ ^
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MS i
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i.
r I
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36
\
\
would be more than fufficient to make them bear the
weight of the fuperincumbent matter, this compref-
fion muft be propagated on account of the elaflicity of
the earth, in the fame manner as a pulfe is propa-
gated through the ^ air
niediately over the c
and again the materials nn
ty, refioring themfelves be
yond their natural bounds, a dilatation will fuceeed to
the compreffion ; and thefe two following each other
}
for fonie time
1
ced at the furface of
bratory motion will be
earth. If thefe alter-
and compreffions fhould fuceeed
iiother at very fmall
they wou
like motion in the air, and thereby occafion a con.-
fiderable noife. The noife that is ufually obferved
to precede
mpany
hquakes, is probably
owing partly to this caufe, and partly to the
of the parts of the earth together, occaiioned by th
wave-like motion before- mention
that firii
th the fire, has formed a cavity
fj. After the water.
came in contaifl:
all the reft of the
water contained in the fiffures, immediately commu
nicating with the hollow left by the part that fell in
reconciled with each other, but upon this fuppofition.
Mac
Fluxions, art. 681, ^c] It appears, from fome late and
accurate obfervations
J"P
alfo, as well as thofe of the earth, are a little higher than they would
be, if their, rife was owing to the centrifugal force, and he was of
uniform denfity ; but if we fuppofe him to be of lefs denfity in the
equatorial, than the polar regions, then the form may be fuch as
he would aflume from the refpe£live gravitation of the feveral parts ;
and any fluid like our ocean, would not overflow the polar parts.
necefTs
would follow his general form, as our ocean does that of the
lar.th..
1
\
*■ r
*%
J
V
\
•37
m out upon the fire, the ileam taking ifs
From hence may be generated a vaft quantity
of vapour, the eflFeas of which fhall be confidered
mull:
prefently
iuppofi
This fteam will continue to be generated
the fire to be fufficiently g
fures before-mentioned are evacuated
the fif-
the wa-
beeins to flow very flowly ; when the fteam
ill be removed b
th
;lallicity of
ready formed v
the earth, which will ag
upon the furface of the melted
lip a little way into all the clefts, by which the water
fubhde, and, preffi
matter, will force
might continue to flow
By this means, all com
munkation between the fire and the water
prevented, excepting at thefe clefts, where the watei
drippin
g
flowly upon the melted matter,
dually for
upon
will g
that will foon flop
farther communication .in thefe places likev/ife^ and
the fifliires, that had been before evacuated, will be
f the water
^
again
radually repleniflied by the
between the flrata
. As a fmall quantity of vapour aimofl: infl:antly
generated at fome confiderable depth below the fur
face of the earth, will prod
vibratory motion
fo a very large quantity (whether it be generated at-
mofl: inftantly, or in any fmall portion of time) will
produce a wave-like motion. The manner in which
this wave-like motion will be propagated, may, in
fome meafure, be reprefented by the following expe
riment. Suppofe a
large cloth
or
upon
fut
a floor) to be raifed at one edg
denly brought down
der it, being
till it
pet, (fpread
e, and then
floor, the air
rrleans propelled, will pafs
fing the
cloth.
again to the
ppofite fid
'^
/a
r
I"
.*■■
^
i
1
;?
— '^
'm
■^
!S
, I
■L*f|
r
r
I
v-^^^
J I- I
I T » t
I
'■■''• 111"
L flk -'■ '\ -
. . 1,1
1
■ "K' Hi
I
I \
t :
,;VlU
li
1l
•i
t" hi] ill),
^Ki]';''i
' I
f !■ h
.i' f
1 »
■Hi
■ ■ r
.1!
iiiihiii
:. !
K
::iF
^ " h I
,-■'
. *
y t
Mm
I J 'H
I
It'iHtii:' .
■1>?l^
Tin i
' ■■■; ;
■ 'l !
I.I, '
4 1
I ' I' I
.ll'l''
^*Hi;ri !
iP.;
■ i^r
f Ll |H I, 1
^>
!;::■: ■::m1
'c
■ 1 1
'.fii/.i;r
.1 m\ i
bii- ' 'I ' i
ms
l..^'...,,,,,
'..'!!
^
cloth in a wave all the way as it goes. In like man
ner, a large quantity of vapour may be conceived t<
raife the earth in a wave, as it paffes along between th(
ill
•ata, w
hich
direction, there being, as
may eafily feparate in an hoi
coheiion between
e
T
ft
ha
faid before, little or
urn and another
The
part of the earth that is firft raifed, being bent fi
form
W]
ill
a
d the
md
par
our to reftore itfelf by
next to it beginning to
ght fupported by the vapour, which
its elafticity,
have their w
v/ill infinuate itfelf under them, will be raifed in their
tiirn, till it either finds fome vent, or is again con-
denfed by the cold into water, and by that means
prevented from proceeding any farther.
59
be
Tf
•ge quantity of vapour Ihould
generated for fome time, feveral waves might
be produced by it^ and this would be, in fome mea-
fare, the cafe, if the quantity at firfl generated was
exceedingly g
though the whole of it was g
rated in lefs time, than whilft the motion was propa
gated through the diftance between tv^o waves.
6o. Thefe waves muft rife the higher, the neare
they are to the place from whence they have
great diftances from thence, they
fource
but
may rife fo little, and fo
as
not to be per
4
ceived, but by the motions of waters, hanging branc
in churches, &c.
6i. The vibratory motion occailoned by the firfl:
impulfe of the vapour, will be propagated through
the folid
par
of the earth, and therefor
It wi
much fooner become too weak to be perceived, than
the wave-like motion ;
for this latter, being
fioned by the vapowr infiauating itfelf between th
/
ata
7
/
-^T1
^
■-H
r
39
ftrata, may be propagated to very great dillances;
and even after it has ceafed to be perceived by the
the fenfes, it may Hill difcover itfelf by the appear-
ances before- mentioned.
S
IV
6
All
earthquakes derived from the fame fub-
terraneous fire, muft come to the fame place in the
fame diredion ; and thofe only which are derived
from different fires, will come from different points
all probability
feldom
of the compafs -, but a
happens that earthqu
affed: the fame place,
that they come from the fame qii
how^ever, to be fuppofed, that this
the cafe, for it will; probably, fometimes happen t<
be other wife : and this is to be expeded in fuel
fed by different fires,
efore find in general,
■ : it is not,
Id always be
cii
places as are fituated in the neighbourhood of feveral
\
•fubterraneous fi
s
or where, bein^ fubjed:
F-
of feme local earthquake of fmall
thev
now and then are affcded by an earthquake, produced
by fome more difl:
but much
fiderable
c
aufe
Of this lafl cafe, we feem to have had fome
1 (lances in the earthquake of the iff of November
/^-f, and thofe local ones, before-mentioned, which
.icceeded it.
63. As we may reafonably infer from many earth-
fame place, from the fame point
comin
q
of the compafs, that they
all derived from
fame caufe, and that a permanent
io we may
fonably infer the fame thing alfo, from their being
propagated with the fame velocity; but thi
s
fiill come with
t2
fo
1
f
argu
be
conn
%
*v#
\
'%\
i^
\
i-n
\
t
^--
I
-.1
n p Jt
^.
fF*-
i|^
t
H 'l
j,r.
{
ml- V^V
I, 'I
LH
liiii:!
'!.■ I
A-
U
t hi-f*!' ; ;.
I ■ I ^
M '■ ■ ■ ' • '-
1' ! '
^m\
ill
L
1 ■-a-vi'i
life-
!!■!!
.ti;'r
[■lil.
!'fflB;
0!;'
m
I I
I. ;[f' f
■ i
. V ■ . . [
ill
^4'
', ,. tM' .
Ii ^>
is
I tk
-%
m'JV t|.
r
I
!*'!i,;;,:,
|i«^*'i
^1^;*
.■;.;:■|I'^J
. ''■*'»
^4
- I
'■\'^
!1
■ I
'IS
..^w.
coniidered, tjiat the velocity of any vapour, which
infinuates itfelf between the ftrata of the earth, de-
pends upon the depth of it below the furface ; for
the deeper it lies, the greater will be its * velocity
We may therefore conclude, from the famenefs of the
velocity of th,e earthquakes of the fame place, that
the caufe of them lies at the fame depthj and from
the inequality of the velocity of the earthquakes of
different places, that their caufes lie at different
depths.
Both thefe are perfedly coniiflent with the
fuppofition, that earthquakes owe their origin to fub-
terraneous fires, iince the flrata in which thefe fubfifb,
may be ealily conceived to lie at, different depths in
different parts of the world.
S E C T. V.
r
\
■ ■ M
. 64. From the fame caufe, we may eafily account
for thofe local earthquakes, which fucceed the greater
and more extenfive ones. If there are many fubter-
raneous fires fubfifting in different parts of the world,
the vapour coming from one fire may very well be
fuppofed, as it paffes, to diflurb the roof over feme
other fire, ar^d, by that means, occafion earthquakes
by the falling in of fome part of it : and this may be
the cafe, in fome meafure, even where the vapour
paffes at fome fmall diftance over the fire ; but it will
be mofl likely to take place, where the vapour eithe
r
The
paribus
am not miftaken) in the ratio of the depth below the furface. This
feems to follow from a known law of all elaftic bodies, according
to which they tend to return to their ftate of rell:, when either di-
lated or comprefled, with forces proportionable to the quantity by
5yhich they differ froni their natural bounds,
pafles
*
_^fl«ME^
-^^
-lUl;
t
41
at fome diftance under it, Or betw
ien tb
jftratum, in which the fire lies, and that next abov
or below it.
le
,V
R
^
a
f
N
Sect. I.
the former part of this trad, I fuppofed
part of the roof over fome fubterraneous fir
n : this is an event that cannot happen merel
dentally 5 for fo long as the roof refls on the mat
fall
fi
part of it can fall in, unlefs
now
below could rife and take its place :
difficult to conceive how this fhould happen, unlefs
y
was
fiffures of the
ife by fome larger pafTages than the ord
th, which feem much too narrow
for that purpofe ; for, befides that the melted matter
cannot be fuppofed to have any very great degree of
fluidity, it muft neceffarily have a hard crufl formed
V
upon it, at all the fillures, by the long continued con-
tact of the water contained in them : thefe impedi-
ments feem too great to be overcome by the difference
of the fpecific gravities of the part that is to fall in,
and the melted matter, which is the only caufe that
can tend to make it defcend^ the manner therefore,
in which, I fuppofe, this event may be brought
about, is as follows :
66. The matter of which any fubterraneous fire is
compofedj mufl be greatly * extended bey
ginal
F
* As all bodies we are acquainted with are liable to be ex-
ten
ded by heat, there can be no^oubt of its being fo in this cafe
F
likewife
\
I- ^ >
k
c
ii
ii
r
1'
*
t
■ d
%
, JT^i
M''
«
;^''i
m
Ik-
iMi
K
k
r
If
1 ^J
\y-
i^\
S '
It, ^:r
[
■ r J
h^i. 1
\ii^w-\:
...Mil!!
iVltSS'!>i''t
L 4
•♦if
■I J
lf^
1 I '
.:"!;;!
:■ ;;i!f;'t
I V- nr r 1. { I
'1*1'. :
' .
■in"
i'f^
I ■
■■'■\
„ ■■■"
Li
i'^^r;
11
/■
\
-*
?mal dimerifions by the he
about
As this will be bi\.^^
/"
row
adualiy, whilfl the matter fpreads itfelf, or
hotter, the parts over the fire will be gradually
raifed and bent : and th
bending
will, for fome
time,
^.^
without any other confeq
b
3
fire continues to increafc
th
earth will
at
as
lail
begin to be raifed fomewhat beyond the limits of
By this me
ed_
fedion of which
r
fire, will be two
ular fpace will be formed at the
the fire, and furround
It,
tical
ace, through
Ions; triandes,
a diameter of the
the fiiorteft fide or
bafe of each
fides
xt the fire, and the two long
being formed by
which will be feparated f
proportionably
upper
r a c
d lov
VQV
fir
fiderable extent.
th
diftance through which
are
fed from each other *.
This
/
they
s will be
gradually
s
llkewife ; but the matter of fubterraneous fires is yet much more
extended, than thofe bodies which are only capable of being melted
into a folid glafs, if we may judge of it from what we fee of vol-
for the lavas, fciari, and pumice ftones, thrown out from
canos
thence, even after they are cold, are commonly of much lefs fpe-
cific gravity, on account of their porous fpongy texture, than the
generality of earth, flones, isfc. and they frequently are even
So-hter than water, which is itfelf lighter than any known follil
fuppofed
bodies, that compofe ftrata in their natural ftate.
* In Fio-. 4. A is fuppofed to reprefent a vertical fedion of the
matter on'^fire; B B, parts of the fanie ftratum yet unkindled j
CC, the two feaions of the annular fpace, (furrounding the fire)
" to be filled with water, as far as the Itrata are
^, .-e feveral fets of earth, ftones, &c: lying over
tbeTre7 which are raifed a little, and bent, by the expanfion of
the matter at A. As it is not eafy to reprefent the things above
defcribed in their due proportions, it may not be araifs, m order_to
prevent the figure here given from miileading the reader, to give
fooie random meafures of the feveral parts, fuch as may probably
approach
feparated ; D
:-ni|Pr
If;;.;: ■
Mi
\
r
\\
cr* r\
-1- ^*- -.
f
rJ
^^ ^.
k'l^
1
43
F
gradually" filled with water
melted matter
as It is formed
the
being prevented from fillin
it, by
its want of fluidity, as well as on account of the
other circumilances, under which it is to fpread
itfelf : for the lentor and flugglfl^nefs of th
kind
of matter is fuch, th
hen fomewhat cooled on
furface by the contad of the air only
will
flow, perhaps, ten feet in a month, though in a very
larcre body : inftances of which we have in the lavas
of ^tna, Vefuvius, &c. It
then, that it fhould fpread far,
tad with water at its ed
I -.
and when it is.
e*^
i not to be expeded
vhen it comes in con-
foon as it is formed.
perhaps, feveral months in acquir
icknefs of a few inches : but it muft, by d
form a kind of wall between the fire and the open
g
before defcribed
Th
is
wall will gradually increafe in height
becomes
too tall in proportion to its thicknefs, to bear any
long
jche prefli
of the melted matter : which
r
^
n.
I
M!
i^!''
:7-\f
i >■■■
f
I 1
4 ,
r
»
w-
i.
r>
r ■-
approach towards thofe which are fometimes found in nature : we
may fuppofe then the ftratum B to be, perhaps, from ten or twenty
to a hundred yards in thicknefs j the greateft height of the annular
fpace C, next the fire, to be from four or five to ten or fifteen
feet, and its greateft extent, horizontally, from ten or twenty to
fifty or fixty feet ; the horizontal extent of the fire at A, may be
from half a mile to ten or twenty miles ; [See art. 29. and the note
to art. 53.] and the thicknefs of the fuperincumbent matter at D,
may be from a quarter or half a mile to two or three miles ; the
number of the laminas alfo, into which it is divided, may be many
times more than thofe in the fip;ure. As to the perpendicular
\
fiffures, they muft be fo numerous, and fo fmall, in proportion
to the other parts, that I chofe rather to leave them, to be fupplied
by the imagination of the reader, than attempt to exprefs them in a
manner^ that could give no adequate idea of them at all.
F 2
mufl
I,
/
1 ^ F
'ft,'
;i' i
!M^
f
%"■
1-1 -T
\
■-
\
\ M^' L'
I I 4 I 1
r .
i I
Mil ■' ■ ^'^
m
III
I'i-ui:!!';!!'!
'(bm^'jKji
•f.i
II
If"! '
" .1 .'(ir; !
■ ,; ^\:
^( -^^^i .1!! .: ,
^^^''■'
r' ;''.
r r
^■,%m
LI
i^iil^M I
: 1
i| .1
■i^[|
r^ :nri I
M
^ ,1 K
^7-
i! ;;(.ir,!
';iv..
'i-
.f
I
^0 1
Vii^ ;
1 ',5Bi|^^;|l!
; I. V' !
'■ I
Li ■ 'I '
h h
m.:i:\
I,
;il:^ i
44
muft neceffarily happen
nefs of it will not exceed
laft, becaiife the thick
67. Befides the giving way of this wall, the fire
may undermine the fpace containing the water, and,
by that means, open a communication between them.
fuppofe
one
thefe come to p
L
time arrived when the partition be
d
yield
If
then the water had any vyay to efcape readily, the
breach would be made, and the melted matter would
s quan-
but as this is not the cafe
burfl forth immediately, and flow
titles at once amongfl it ;
and it can only efcape by oozing flowly between the-
ftrata, and through the fiiTures, the way that it came,
the breach will be made gradually, from whence we
may account for fome appearances that have preceded
great earthquakes.
68. We are told, that two or three days before
an -f earthquake in New England, the waters of fome
wells were rendered muddy, and flank intolerably
d
* This limit will depend upon the thicknefs cf
r
fary to prevent fo quick a communication of the heat pr cold
through itv as that the water fhould be able to diminifh the heat
of the fire confiderably,.. The thicknefs requifite to do this, is very
different in different kinds of bodies. Metals of all kinds tranfmit
heat and cold extremely readily y_ but bricks and vitrified fubftances-
(with which laftvve may clafs the matter under our prefent confi-
deration) tranfmit them very flowly : the walls of the hotteil of our
furnaces, when built of bricks, and eighteen inches thick, will not
tranfmit more heat than a living animal can bear without injury^,
thougb the fires are continued in them for ever fo long a time;,
probably, therefore, if we allow two feet for the thicknefs of the
matter, cooled and rendered hard by the contad of the water, we
fhali not underdo it.
p. 68g,
Tranf. N" 437. or M
km. vol. viii
V
•■- ".!•
^
Lh II
I L
•li
-%
i\
■
r^
impre
why might not this be occafioned by the water
tained in the fpaces before defcribed, which, being
fulph
fleams, were driven
up, and mixed with the waters of the fpring
leaft, there can be no doubt, by whatfoever means
was brought about, that this phsenomenon was owi
to the fame caufe, already beginnino; to exert
which afterwards
rife
felf,
the fucceeding earth-
quake.
6g
w
Something hke this happened before the
reat Lifbon *
told,
then
that at Col
h quake of
7SS
about twenty
cc
We are
miles from
I I ft of No-
(S
cc
<c
<c
(C
cc
cc
cc
cc
cc
cc
tt
cc
(C
mber
eafed
the afternoon preceding
e water of a fountain was greatly de
1 the morning of the i ft of Novembe
itr
very mudd
d after the earthquake, it re
nels.
ned to its ufual ftate, both in quantity and
The fame author fays, a litde lower
cc
m
the afternoon of the 24th, I was much appr
iive, that the following days we ftiould h
other
:h quake
for I obferved the fame
pro
tha
noftics as in the afternoon of the 31ft Odob
is/' &c.
" And I farther obferved
the
water of a fountain began to. be difturbed to fuch
degree, that
ht
of a yellow clay
colour; and from midnight to the mot
of th
cth, I felt five fhock
of which feemed
*« me as violent as that of the 1 1 th of December."
70. But the moft extraordinary appearance of any
that preceded this earthquake, was that of the a
-4
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' ■ L
i .'
M
t
i
J h
!i^
I
'^>
m
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V.
i!^
\
*
See Philof. Tranf. vol. xiix, p. 416 and 417,-01 Hift. and
Pfeilof. of Earthq. p. 313
tioa
\
t .
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1
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( :ji ill" "
L I
1h 1^ t' ■ 1 1
I 11 I ^'11
u
'if I
'■It
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t. ti
it
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i -
iiiilsiffl'i
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•1 1;
Si!
n hi
iltiimillllii
i
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46
-tion of the waters of * Lochnefs, and fome others of
the lochs in Scotland, about half an hour before any
motion was felt at Lifbon, notwithflanding the caufe
of all thefe great effeds could not lie far from thence,
and, I think, certainly lay to the fouth of Oporto.
Nor is it probable, that there fhould be any miftake
in the time, not only becaufe the difference is too
great, as v/eil as the concurrent teilimonies too many,
to admit of fuch a folution; but becaufe they men-
tion another greater agitation, that happened about
an hour and half after the former ^ which latter agrees
with the times, when the agitations of the waters
were obferved in England, if we allow only a proper
interval for the motion to be propagated fo far north-
Ward, proportionably to the time it took up in tra-
velling from its original fource near Lifbon.
71. Thefe appearances feem to be connecfled witk
that mentioned in the preceding article, and they
may both, I think, be accounted for, by fuppofing
a confiderable quantity of vapour to be raifed,
whiifl: the partition before-mentioned was begin-
which time, a partial
nin
to give way
during
r
* See Philof. Tranf. voh xllx. — or Hift, and Phllof. of Earthq*
art. Lochnefs, Lochlommond, &"€. .The fame thing alfo feems
to have taken place in Sv/itzerland ; for Monf. Bertrand fays, that
all the agitations of the waters in the lakes there, which were ob-
ferved on the ifl November 1755, happened between nine and ten
in the morning; and particularly at lake Leman, he fays, the agi-
tation happened jufl: before ten ; which, allowing for the difference
of longitude, muft have been juft before nine at Lifbon ; and, con-
fequently, if there is no miPcake in the times, all thefe agitations
preceded the earthquake, at this laft place, by near three quarters
of an hour. [See Memdres fur les trembkmms de Terre^ p. 107
J
communi
I
I
"^
J
. \
r
— - -1
«^ A,
I
.^
/
47
V
communication between the water and fire would b
u
broug
d that by de
only
H
but
vapour,, noi being produced at once
might creep * filently between the itrata
ally,
'ards
that quarter where the fuperincumbent mafs of earth
was
li
tteft; and
means, fome places very
fource of the vapour mi
ht be
at all,
fFecled by
affeded, thou
whilft others mi
ley lay at a g
ht be
diftance
thofe places, which lay immediately
part where the vapour was paffin
ny efFed
of the
motion
ccafioned by the fmall quantity of
or not
reatly
\ and
r the
might not per-
fs of the
it. This
^
cafe
might continue to be th .
country where- the fet of ftrata above beiri
came to fom
much
thinner, the vapour would not only be hurried fo
r
ward, but colleded alf
u
pafs;
prodn
and therefo
mor
ch narrower com-
raifing the earth more, would
and this we ought
^
fenfible effeds
* Some appearances that have been obicrved in New Eng.and
feem to confirm this, and make It probable, tnat a fmall quantity
oftapourl often found to creepfilently between the ftrata, before
aUneral communication between the water and the fire g.vesnfe
toihe ..reater and more fenfible efFe,as of earthquakes. See Philof.
or Martyn's Abr. vol. viii. p. 693. where we are
Tranf. N
told, that, at Newbury, a little beforeany noife or fhock was p^-
ceived, the bricks of an hearth were obferved to nfe, and, falling
to lean another way. In the fame account, it ,s aifo
a few minutes before any ftock came, many people
• ' ir ftomachs:' an eftedt.
down asain, to lean another way.
faid, that "
«^ could foretell it by an alteration in their liomachs : an ettect,
wlSh feems to be of the f^me kind with fea-fickaeft^, and waich
S:;ays ac:;Lpades the wave-like moti^. of earthquakes, when .t
is fo weak, as to be uncertainly diftinguubab.e.
J'
cliieily
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n
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48
/-
P
7
fe
Hy to exped in the moft mountainous
idea before given of them
ord
To
in Fi
ake this fomethin
ft
I.
th
t>
d,
in the dotted
vapour to be paffing b
fup
C
d
d
A : whilft, the
ap
t^
P
/^ «*
r
little,
parts at E, it
IS well becaufe
aife the
forward
paffes under tl"
arth over it bi
be fpread broader and
wei
rives
becaufe it will be more comprefled by
of the fuperincumbent
b
as It ar
ds A, not
the latter
P
w
driven forwards with greater velocity, b
moft will travel flower, on account of it!
ita : and, beiid
be
fore
under a 4- thinner fet of ftr
d being much lefs
It wi
From all thefe caufes taken
tly expand itfelf.
furface of the earth, occafioned by the paffing of
vapo
nder
will not only be much h
and, confequently, the ;
jnts, will be much mor
d, mor
b
aifo much fliortei
.on both thefe ace
to the horizon: i
of th€ wave will be flower, it wdll sive moi
^
u
becaufe the p
led
efs
any waters fituated on
fide of
flow
one
way J and on this account alfo, the apparent ag
of them will be increafed.
Sec t, II.
73. We are told, that, in the Lifbon earthquake
of 175-5-, "the bar [at the mouth of the Tagus] was
" feen dry from fhore to fliore 3 then fuddenly the fea,
* See art. 43,
+ See art. 63. the note.
<c
like
V
^
A
i ^^^ :^
J ^ I
49
ec
cc
<c
cc
•cc
cc
like
came
in
and about B
lem caflle, the water rofe fifty feet almoft in aii
ftant
and, had it not been for the
bay
ppofite to the city, which received and fpread
le great flux, the low part of it mufl have b
under
The fame phenomena were ob
ferved to accompany the fame earthquake at the ifland
of Madeir
where we are told, that, at tl
le
city of
Funchal, " the fea, whicn was quite calm, w
/
cc
cc
cc
cc
cc
to retire fuddenly fome paces; then rmng
ferved
with a great fwell
fuddenly advancing
leaft noife, and as
flowed the fhor
d
ed the city. It rofe full fifteen feet perpen-
dicular above high-water mark, although the tide
et, was ther
f the ifland
^ I
" which ebbs and fiow« there feven fe
«* at half ebb. In the northern part o
V
cc
cc
cc
cc
cc
the inundation was more violent, the fea retiring
there above one
hundred paces at firft, and fud
denly returnln
verflowed the fli
for.„.^
open doors, breaking d
the
alls of fe
ma
^
d florehoufes, and carrying away
** its "recefs, a conflderable quantity of grain,
fome hundred pipes of wine f
ppearances
d
74
B
thefe
which hav
I
obferved to
d fe
other
thquakes
as
11 as this) feem to admit of an eafy fo
fup
poflng the caufe of them to lie under the bed of
ocean
the farther progrefs of the communi
for.
between the fire and water, the vapour, th
IS
* See Hift. and Phllof. of Eartbq. p. 316.
t See Philof. Tranf. vol. xlix. p. 432, ^^V.— or Kid. and Philof.
.©f Earthq. p. 329
G
^
gradually
^
li
i
r
I
iii'.:
I
X
^
4
i
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k
H
I, t^
4
''^.
V
■] -
i-
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4.
t
t
i I!
■|
r. ^
! iij^iv-fl'^ji
' ijit-
'iff
■'
■iii
1 bi:^*ft':iii
1 il^' "^^■■" ^
5 n .^
L fib ^II
^ I 9
"^*q
I-
MM
' mtar:"- -,
t \^
'Ih'i
1^ "ll'-
L_ It
V:, I
i'IHi
"_l !■ I
Hj^;,l., .^J ll'
n
I I ■
t
I k
m
i'»'
cfvi'
I ,
t
'^«^: J^
~rU'' >.,'rt
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"ill' l''*,»l''
] ■
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Ik
,1 ^**
I r
L L ■
h
■ ■ :;tl
gradually raifed at firft, will at laft begin to raife tlie
oof over the fire, v/hich, being fupported by fo light
vapour, there will now be no want of fluidity in the
matter it
fts
upon
d the diffe
gravity between the two
of fpecific
of being fmall, will
be very g
hence, if any part of the roof gives
way,
rifin
muft immediately fa
d
4-
aking its
once made, a
the vapour readily
nd a beg
b
com
berlefs clefts and fifli
ation v^ili be opened with
falling in of vail quantities of
ft OGcafion the
^/hich, as foon
the vapo
pafs round; them, will
t
the
fupportj then will follow ...^ ^
defcribed.
/
y^. Now, whilft the roof is railing, the waters of
the _
thence
lying
y way
muft
this, howe
and flow from
being brought
J
about flowly, they will have time to retreat fo gently
^s to occaflon no great difturbance : but as foon as fome
part of the roof falls in, the cold water contained
the flflures of it, mixing with the fteam, wi
V-
J^
diately produce a vacuumy in th
fame
manner as-
/
water injedled into the cylinder of a fteam eng
d the earth fubfidins^, and
g
a h
w pi
r-
above, the waters will flow every way towards it,
and caufe a retreat of the fea on all the fhores round
>
abou t
then prefently, the waters being
am con
erted by the contad of the fire into vapour, togeth
h all the additional quantity, which has now
open communication with it, the earth will be raifed
and the waters over it will be made
^—
flow every
/
'^ See art, 56 to 60 inclufive
l-r
/
wavj
i
and produce a great wave i
ing the previous retreat *.
Sec t.
quantity of water, which we h
upon.fubt
fuppofed to be let
by that means, to produce earthq
us with a reafon, why they obferve a fort of p
cal return. This water mufl extinguifli
tion of the burnin
por
^
matter
d though th
onfequence of which,
a(5ted within much narrower bounds ;
could not
/
effeds before defcribed
heated
place at firfl, but by the great exten-fion of
yet, after they have once taken pi
ii
S;
'-,
"I .
1
H. [iJ
m
f
y
* It may, perhaps, be objeifled, that thefe pKxnomefia may as
eafily be occafioned by a vapour generated under the dryland",
which, by firft raifmg the earth upon the fea-fhore, would make
the watervS retreat i and that the return of them again, upon its
fubfiding into its place, rnight caufe the fubfequent wave. That
this may be the cafe, in fome inftances, is not impoffible, but, I
believe, upon examinijig the particular circumftances, it will ge-
nerally be found to be otherwife ; and there cannot be any doubt
about it, in the cafe of the Lifbon earthquake^ for the retreat w^as
obferved to precede the wave, not only on the coaft of Portugal,
Mad
now
if the retreat had been caufed by the railing of the earth on the
coaft of Portugal, the motion of the v/aters occafioned by this
means
Mad
there previous to the retreat, contrary to what happened ; nor
ould the motion of the wsters at Madeira be caufed by the earth*
<]uake at that place, becaufe it did not happen till above two hours
been
continuation of a motion propagated from the place, where the
earthquake exerted its firft efforts, ^ind we may obferve, in gene-
ral, that this muft always be the cafe, whenever the retreat does
not happen till fame confiderable time after the earthquake.
G
2
they
/
I
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- 1-'
h
V
1 1*.
1.1
"»
■<'
I
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:-f
■'i
'i^^-^'M
- \
IS
1 .■^L:
'■ iiiT,
■ ^ftiiiiiii"'"''
'^VAP^
'.]'.■
iiii
l'^
^^ii
M. I
■^^ii
T ■ il'.
lift. IV- "i-*^
rh
vj.
' ,! 'X\
'f'V.',..M[
^
!:i|i.
■' Y
If- I L
■ I '. . "'
■ -J
HT*^"^ -til
r
difturban
falling in of
frequent communication
do fo for fome time
inftance,
part of the roof, muft render the
fire and
ly very eafy, but almoft unavoidable
d this
will continue to
roof
f(
and the furTace of the melted matter
cooled, after which
may require
for
the fire to heat it again fo much, as will be necefiTary
make it produce the former effed
Now
th
e
matter has been m
cooled
the com
buflible materials are with more or lefs difficulty fet
fi
t)
of other circum
/
ftances, the returns of thefe effeds will be
earlier i but though they will not, for this reafo
obferve any exad period, yet they will generally fall
within fome fort of lim
either the matter th
occafions them is confumed, (which, probably, will
feldom happen in lefs than many ages) or till the
fires open themfelves a palTage, and become vol-
canos.
r
Sect. IV.
E
77, I have already intimated, that me moil exten-
five earthquakes frequently take their rife from the
lea
Accord
the defcription of the * ftrud
of the earth befor
fi:
g
y combuftible fi;ratum
at
depths in places under
/
than elfewherej, hence far more extenfive fires may
fubfifi: there, than where the quantity of matter over
them is lefs^ for any vapour raifed from fuch fires,
* See art. 43.
havin
\
'Am
■ ht '
. ^■'.■rt»l
1'^
\i
m
It
53
having; both a flronger roof over it_, and bein
by
wei
felf,
b
e
preiled
hC (befide the additional Vv-eight of
w^atTrTwiU not ordy be lefs at liberty to expand
and confequently of lefs bulk, but it will alfo
filv driven away towards the parts round about,
here the fuperincumbent matter is lefs. and th
rp'
1 112
fo
from
light
fo ei
liter.
fires,
&
On
whe
other hand, any vapour raifed
th
fuperincumb
matter is
findin
eafily d
cd heavie
pen a m.
weaker roof over it, and bein^
ii
away und
ft
that are
thick
will be very apt
break
o
er
d
ons; before the fi
id it muft neceffarily do
have fpread them
/
fufficiently, to be near equal
fubfift in places that lie deep
to
ofe which in
All this feems
y
be greatly
which are
firmed by th
fit
f
ft
ys
mountains, and thofe of
the world
found on
fome of
the
*
tops
g;heft
of
7
8. If, then, the- largeft fi
fubfift under the ocean, "
It is
be fuppofed
der that the
-»^
r^
* Perhaps this may fupply us with a hint (if the cdnjeaure is
not thought extravagant) concerning, the manner m whicn thefe
mountains have been raifed, and why the ftrata he generally mo.e
iaclinino- from the mountainous countries, than thofe^countnes
themfelves; an appearance not eafily to be accounted for,. but
^ the ^ppofition, that the upper parts of *--«^,-^^P-
matter, in fome degree, though not perfedly fluid and that .ni.
ma te is hghter than^the earth.that reSs upon it. Th. conjeaure,
however, will probably be thought lefs firangc, if it be conhdered,
•thauhe new iLds, formed abSut Santerini-and the Azores have
fome of them been raifed from 2C0 to 300 yards, and upwards ; a
height which might well enough intitle them to the denommaticn
of mountains, if they had been raifed from lands not lymg under
the ocean. [See Fig. 3.] -_
moft
^
i
J ' ^
• >*■ .
•^^
i L
4
^],
I .-
111-.;
■'.?i
HI
I?
k
a
1
L
i'
1
>
It-;
\
t
t
-I
I
Hf
1 ■-_ ■ r^
■ =■ I
I
" I
H h
'1^
1^
I, h
i\
Hi
"i
r 'T ■'. nil
I ►
:;;i
p.
!l ^^"■'"'Wll.
I >■ I
Ik
I . I 1
I?
■ f . ^'
n
:':.J^^'t
\^m:p
I ' I
u
\
thence
th
54
five earthquakes (liould take their rife I-otn
e
reat
iliewn to have done fo
hq
ike of Lifbon has
d that the caufe of
been
alio at a greater depth, than that of many others, ap
prop
7<
from
<yated
•f- velocity with wl
t>
ine
reat
d Callao in 1746, d
thquake that deftroyed Lima
to
h
/^
from
f
a
for feveral of the ports upon the coaft wer
^ f
overwhehned by a great wave, which did
four or five minutes after the earthquake bepan,
which v/as preceded by a t retreat of the water
and
as
Lifbon
A
be alleged, that there were fo
fuddenly, in the neighbour
nfl: this, it may, per hap
mount
thquake happened, and th
the B
broke out
when this
of thefe
^... be the
*^
cafion of
This however, I think
IS
y probable; for, to
om
wave, and previous retreat of the wat
y likely, that mor
the argument of
mentioned
fire was concerned : befi'des
felf a
,1
pofed
pafiage at thefe pi
already
an one
pour, opening it^
Id not wxll be fup
far ;
it took its rife from thence, to fpread itfelf
efpecially towards the fea, where it is manifefl
N
)
4 ,
till
"1-1
'■Mm-
v
if'
i^.
\ P
?*!!!;
^^^|i; '^
'ml
I "i™ ' ■ h, ! L
* See art. 54
t See the note to art. 63.
;|; Both the wave am
er great earthquakes
1,. \
L
See alfc) art. 94 to 97 inclufive.
Other great earthquakes^ which have happened at Lima, and m the
nnghbouring country. See d'Ulloa's Voyage to Peru, part ii.
book i. chap. 7,
If thefe volcanos v/ere not new ones, but only old ones which
^^ _^ _ J ■■■■■■ ■ ^i^^ ^
with fiill greater force.
[See the note to art. 34. 1
/
r
that
ii?if>i
V
"t
\
\
tliat the flrata over it were of great thlcknefs, as a
pears from the great velocity with which the earth
tinued with equ
propagated
I
ly equal
months after the openings were made 5
fires had been the caufe of them
for fome
hereas.
ceafed,
happened
more probabi
other
immediately
a vent,
therefore m
quantity of vapour, takii
more exteniive fire under the fea, fpread itfelf fi
fires finding
cafes.
y large
me
then
as It
!-/=»
over it was naturally much thinner, as well as greatly
weakened by the undermining of thefe fires, it opened
itfelf a pafiage. and buril forth
lo. As the moil extenfive earthquakes
snerally
proceed from the loweft countries, but efpecially
fi
,e
fea, fo thofe of a fmaller extent are g
lly found
i
ft
hence it almof
always happens, that earthquakes, which are felt
fea, if
felt alfo in
lands
where
th
gh
and thofe
y
are many amongft the hills,
extend
violent ones, which never
tiemi
the lower countries.
T
we are
told, th
a
t<
■**
the
Jamaica, " -f fi:takes often happ
try, not fe
Port-Royal
d fome
times are felt by thofe that live in and at the foot
* See art. 28.
Th
J
that hap-
before- mentioned, was attended with the Vv'ave and previous re-
iiiL N"" 209. or Lowthorp's Abr, vol. ii.
treat.
of. Tr
7
£C
of
i
H
/
N
' h
.*
Tl
P\
*■
i-'
jiii
f;
xu
h
I
i
.11
Si
,r
1'
■w..
%
V,
I'!
■■*:
/'
^.
■-
f-
W
. IT
t .
„.:j
- ' ■■;
\m-A.
■(■..iifi*^'
r '
H l>
t'
k>
■:".iii
ii
ir
I ■
. I
mm
^1 ■.
il%»vf«
» h
, ■ ■ .■ 1
.■ti
^^(1
llirfTilkiii
i;f
;J
>
r.
■i!
!i:^
,'-i:
I r±
* I
; >
-,Li-i
111". 'Wi
Vf.'
/
L L'
ih
i¥
\
o
R
ther
mountains, and by no body eife." On the
thquake - that defcroyed Port-
d
the
ded itfelf
over the illand
d the
\
/
fame was obferved of a fmaller earthquake, that hap-
ch latter undoubtedly
by Sir * Hans Sloane's
P
ned there in 1687-83 wh
ame from the fe
of
app
p. in f*
Lai
Si. Earthquakes of fmall extent are alfo very
mon amongfl the mountains of Peru and ChiH.
An
CQ
d'Ulloa fays, ^^ Whilft we were preparing for
cc
<c
cc
cc
cc
cc
fe
fou r
our departure from the mountain Chichi-Choco
there was an earthquake which was
leagues round about : our field tent was tolTed to
and fro by it, and the earth liad a motion like
that of waves: this earthnuake, h
was
of thefmalleftj that commonly happen in th
; author tells us, in anoth
Luace,
P
cc
untry. '
that.
The fam
during his flay
H._
th
or
in the neighbourhood of
ther
ity of Q
f^
WTre two
quakes, violent enough to overturn fome
(<
cc
nd
the country, which buried feveral perfons
yy
V
Sect. V
8
it is generally found, that earthquakes in hilly
countries,
much more violent than thofe, wh
h
a
PP
n
wh
d th
s is obferved to be the
cafe,
as
well when they take their rife from th
lower countries, as
amongfl
tl
le
h
th
em
fel
ves.
This app
from the ftrudure of
being fo eafily to be accounted for,
already defcribed, I
earth
1
^ See Pliil, Tranf. N° 2093 or Lowthorp's Abr. vol ii. p. 410.
x
Ip
^1
Ii
\
I k
y 9
»,■
-I
^jJ
-^^
n
*.
-^-
^
57
fliall content myfelf with eftabiifhing the certainty of
a faa, which tends fo greatly to confirm *
8
Th
c
hquak
h
infefted fome of
the tov/ns in the neishbpurhood of Qi
have
^Jy been incomparably more violent than that v^hich
delkoyed Lilbon, but they feem to have exceeded
that alfo which deftroyed Lima and Callao. ^ In
* Lifbon, many of the houfes were left ftanding
1th
h few of them wei
lefs than four or five
At Lima alfo, it is only faid, that
"all
reat and fmall, or at leaft; the
Callao
ilories high.
" the building , ^
** greatefl part of them, were deftroyed
likewife
_^ ^ ppears from the accounts we have of
it" had' many houfes left unhurt by the earthquake
the wave came
which overwhelmed the whole
town
way.
thofe produced by
Its
and threw down every thing that lay in
All thefe effeds feem to be greatly fhort of
hquake that happened
Latacunga, in the' year 1698, when the whole town
cbnfifting of more than fix hundred houfes
was
minutes time, a
..:ely deftroyed in lefs than th
part of one only efcapingj notwithftanding that the
houfes there are never built more than one ftory high.
\
order, if poflible
avoid thefe dang
Am
bato
ge about the fanie fize as Latacung
gethcr'with a great part of Riobamba, another town
in the fame neighbourhood, were alfo entirely de
ftroyed
the fame
and fome others
were either deftroyed, or received confiderable damage
'* SeePhilof. Tranf. vol. xlix. p. 403- where it is faid, « of
«« the dwelling-houfes,
« that tumbled."
there might be about one fourth of them
V
i-
n
/
II
•1
\f.f
!
5
I
I
f^r
!!>
■It;
i'A
/
;lf-
>
i
I
I . ■
'1 H
' ^ :
11
I :
i!
' -t
I;
JT
L
,+^
}
jr
iil
It,-
|!>
\i
*
f r
•I
y
\
\
/
V}
b -%
-«l_l*'
;^ . '^i'-l ^i|^[k
I !
J I ■
' . ^
111
i
;Jll
r r
>i'
\- ,
... Q
. J ft ri ' Lif. I ' .1 (
"r!
w
'I'l
■iilf''«iii''r.-'"
■)1
^ u'>'
\\
'.ii
-ll^-s^i
/
■r
■)^''
I 'i f
'.'W^m
■ :L
I
r
■ii\ J:'
'i.
■;;;;;,,w.-':
ii
F t
I I
Ki;
/
f
^'*^
-'
flenly ,n the neighbouring mouniain of Carouavrafo
as before-mentioned : and. " ne„- A.^uJ'iuJ'^^",
€C
<c
cc
CC
before-mentioned
opened itfeif in feve
mains
Ambato, the earth
the fouth of that town
five feet broad, and about
and there yet
cleft of fo
g
gth
lying n.th and ibuthMhe,;;;:^!,,^^^^
of * Quito was affefted
ed no damag
the farhe time, but
two geographical miles from L
■-^^^ «. uxiw mnic ume, but re
though It is no more than forty
whe
have exerted^itfelf
far fr
the greate^ft violence of the /hock feem. ..
Thefe towns are fuppofed to
being
fland by far the higheft of any in the world oeins
of &S *' '"^- ''^l^'^"^ the tops.f S
01 tUe higheft mountains in Europe ; and the emu,,^
upon wWch Riobamba ftands, 4ms but f^f
fift n?o?? *"%'rf ^ ^^ ^'S" '' Snowdon th^
•njgnelt mountain in Wales.
it .T^^ ,'^'';^"^7 "P^" .wl^^ch thefe towns fland,
another fet of
mountains, which are much the
ferves as a bafe, from whence arife
high
ds
d
higheft in the known world
tains there are no lefs than fix
Amongft thefe
if not mor
Tho
* The city of Quito ftands lower
by about 500 yards perpendicular. . „ou2n it elcaoed thi, \t
.feas lately, however, been deftroyed by anothfr v olent Lr, Ji'^'V
tiiat happened on the 28th April 1756'TS T h 'fnl? '
reen any other particulars worfh notice. ' ^'^'''°'-
clZ!!!JL!^1i"g to Antonio d'ailoa*. account: burMo
a%
«.umpwiiigita!ti77otoifcs
meridian.l
[See his meafure of a deg
^J
^
within
^
V
i
\: ^-
J-
f *■■
\
59
within an extent of 1 20 miles Ions:, and lefs than
thirty broad
loweft of which exceeds the heig
of Riobamba by above two thirds of a mile, and the
ighefl: by more than twice that quantity
Now
as
hquakes have been more violent at the foot of
thefe mountains, th
in the lower lands, fo they
have been ftill more violent towards the tops of them
this is fufficiently manifeil, from the many * rents
made in them, and the rocks that have been broken
off from them, upon fuch occafi
ftill more manifeftly, and beyond all difp
but it appear
burfting forth of
/
the very 4- fummit of the
ite, in the
■
which are almoft alway
found
where they
In thefe inflances, the earth, ftones, &c
which lay over the fire, are generally fcattered by the
violence of the vapour, that breaks its way through,
to the diftance of fome miles round about,
Bf. The great earthquake of the ift November
^7Sfy was alfo more violent amongft the mountains.
€C
cc
cc
#c
cc
ct
<c
the city of Liibon. We are told, th
<c
the
mountains of Arrabida, Eftrella, Julio, Marvan,
and Cintra, being fome of the largeft in Portugal,
were impetuoully fhaken, as it were, from their
very found
their fumm
d moft of them
pened at
ts, fplit and rent in a wonderful man
ner, and huge maffes of them were thrown down
into the fubjacent vallies t." . -
I
t
-'-^
H
'i:*.
I ■ ^
J?;
'ii-.
I.
\¥.
V-,
i\
')t
if =
I
^
.;■'
f.
^
t
*
k
y
- 4
Per
•-_ \
t The only exceptions that I know of to this rule, are in thole
cafes, wliere the high eft part having an opening already, fome
frefli mouth opens itfelf in the fide of the mountain.
± See Hift
H 2
86. The
^
\itr
;. J
I*
II ^
if
■■■■:-r a
> ,"- J l\.
I \
I' ;
!fl
•>
M
.1..', ri
h'
H +
I ^ ip
H'
i!t^aji^;C
r ^ .11
^^;»n(|ii'
l^'
'^!**'';fej
r'"M
Jill
IM ■ Jti!
II
'I
mm:
.;tv:[J I
1 \
p,!!?^^^!'^!:!!
r^H" ■'■:
mM<
I'll
ii
f ■
i-** :'S■^*^
If ^1.
d'M
■ ' . J 'P *' '
«,&■'!■
^li 1
,*;
"=19^1 I »
ct
T ^U
Hh
'fl
: ^5(;.::^
ll Tfc
»1 I
'k
V
6o
^ 86. The fame was obferved at Jamaica likewife.
In the earthquake that deflroyed Port-Royal in 1692,
we are told, that " more houfes were left ftanding
" at that town, than in all the illand belides. It was
cc
r
<c
cc
r
U
iC
U
\
<c
«
u
arms fpread out, to prevent being
the incredible motion of the
fo violent in other places, that people were violently
thrown down on the ground, where they lay with
their legs and
tumbled about
earth. It fcarce left a planter's houfe or fugar-
work ftanding all over the ifland : I think it left
not a houfe flanding at PalTage fort, and but one
** in all Liganee, and none in St. lago^/except a few
cc
<c
cc
<c
cc
cc
low houfes, built by the wary Spaniards. In Cla^
rendon precincft, the earth gaped, and fpouted up,
with a prodigious force, great quantities of water
into the air, twelve miles from the fea: and all
over the ifland, there were abundance of openings
of the earth, many thoufands. But in the moun-
^tains, are faid to be the moft violent fhakes of all)
** and it is a generally received opinion, that the
it
U
u
cc
€C
CC
nearer to the mountains, the greater the fhalce^
and that the caufe thereof, whatever it is, lies
there. Indeed they are ftrangely torn and rent,
efpecially ^the blue, and other highefl mountains;
which feem to be the greateft iufFerers, and which,
during the time that the great fhakes continued,
bellowed out prodigious loud noifes and echo-*
mes
87'
cc
Not far from Yallowes, a mountain,
having made feveral moves, overwhelmed a whole
family, and a great part of a plantation, lying a
mile off 5 and "^a large high mountain near Port-
^* morant
v ^
■r.'-
(
N
/
cc
cc
morant, near a day's journey
quite fwallowed
is faid to be
88
({
In the blu
from whence came
(C
cc
cc
cc
cc
cc
thofe dreadful roarings,, may reafonably be fup
pofed to be many ftrange alterations of the like
nature -, but thofe wild defart places being very
or never vifited by any body, we are yet
g
of what happened there; but whei
cc
cc
they ufed to afford a fine green profped, now one
half part of them, at leaft, feem to be wholly de-
prived of their natural verdure *."
s E c T. yi.
89.1 have fuppofed, that fires lying at the greatefl
depths generally produce the moft extenfive earth-
quakes, we muft, however, except from this rule
thofe cafes where the depths are very great : for, as
the weight of three miles perpendicular of common
-earth is capable of abfolutely repreffing the vapour of
inflamed gunpowder, fo we may well fuppofe, that
T
* See Philof. Tranf. N** 209. or Lowthorp's Abridg. vol. ii.
p. 416, i^c. where there is a great deal more to the fame purpofe.
See alfo Hift. and Philof. of Earthcj. p. 286 and 287.
From the authorities quoted in this fe<9:ion, it appears, how
the notion, that either large cities, or
towns fituated near the fea-coaft, are more fubj eft to violent
earthquakes than others : it is not, however, much to be won-
dered at, that fuch a notion fliould have prevailed, after the great
deftru(aion that happened in fo large and populous axity as Liibon ;
fmce the demolition of a few ruinous houfes only, in fuch a place,
would have afFeded the imagihations of men more, and would
have been more talked of, than the fubverfion of whole mountains
in fome wild and defart country, where at moft half a dozen un-
feel the efFe<Sts of it, or perh
diftance
ther
, ^^^^
L. \
/'
'
I
i
I
N
I
a
/
I
I
i
Ft
.*
I--
4
!.T
\
ii ^ff'm
\m^^m^
'^''■■}'^
i ' I
>
^ U i ■
■M'^^M
h -
'1 =
t I
;ii' ^'
■ ^
ll
h
L
ll
ii:'
k;
r.
)\'.i^
■
\-
w.
f"*.l., til
ar . . .
It!
II'
Mr"'
I ll
i. ih
,t . t\<:'
'if
■W
^ I
I
be a quantity of earth fufficlent
vapour
ever tins is the cafe
f
h ever fo much heated
d keep it within its orig
fFed:
manifeft, th
Now, when
or,
may happ
though
pro
quantrty of earth may not be fufficient abfolutely to
rcprefs t!ie japour, yet it may be fo great, as to fufe
pand but very
m this cafe
quake arifing from it would be but of fmall
earth
hke motion would be httl
e or none; the
and
Vibratory motion would be felt every-where ; anc
the propagation of the motion would be very quick
rhis laft cu-cumftance being almoft the only one
by which there earthquakes can be known froi thofe
which owe their origin to fhallower fires, it muftbe
diftinguilh them with certainty
very difficult
it is almoft impoffible-Jo diftinguifh'ihe mlZ'c, of
the time of their happening in diiFerent places, when
the whole, perhaps, is comprehended within the fpace
of two or three minutes, poffibly, however, feme of
the earthquakes, which we have had in Engl nd may
have been of this clafs. S^diia, may
Sect. VII.
90. If we would inquire into the place of the
n of any particular earthquake
lowing grounds to go upon
g
we have the fol-
Firfi, The different direaions
i
feveral diftant pi
the
in which it
thefe direction ^
fediion muft be
liable to great difficult
if lines be drawn _,
of their common inter
arly the place fought : but th
moie to great difficulties 3 for there muil necelTarilv
be great uncertainty in obfervations, which cannot 7t
beft
J
I
r
r
>
r
)
&3
beil:, be made with any great precifion, and which
rally made by minds too little at eafe to be i
obfer
of what pafles
mor
themfelves may be fomewhat varied, by the
the diredion
qua
e
the weight of the fuperincumbent matter
der which the vapour paifes, as
caufes
we
as
other
<)2. Secondly, We may form fome judgment con-
cerning the place of the origin of a particular earth-
quake, from the time of its arrival at different pi
t,„i. *!,:„ _ir_ ?- !• 1 1 . ,.^, . . -T
but this alfo is liable
thefe method
great difficulties. In both
g
^eg
however, we may come
of exadnefs
amongft a variety of
different obfervers.
But
to a much
taking a medium
they are related by
93. Thirdly, We may come to the greatefl degree
of exadnefs in thofe cafes, where earthquakes have
their fource from under the
fiance
for
thefe
the proportional diflance of different pL...
from that fource may be very nearly afcertained, by
the interval between the earthquake and the fucceed-
ing wave ; and this is the more to be depended on
as people are much lefs likely to be miflakcn in de-
termining the time between tv^ro events, which fol
low one another at a fmall interval, than in obferving
the precife time of the happening of fome finp;le
event. °
- P4. Let us now, by way of example, endeavour
to inquire into the fituation of the caufe, that gave
nfe to the earthquake of the ifl of November 175-5-,
the place of which feems to have been under the
ocean, fomewhere betw
and Op
(though probably fomewh
the latitudes of Lifbon
' to
the
•^■:
•y
i
I
J!
' ^1
w -
!
■
A
•'■ I
;,'
1
I
I n^
!i)
i
I
\v
T
(
;'J
t- ±.
; if!h '^i ■ - I
: 'rrl> !ii.Ti4:i''i-;|; k ..
Tf ■
.^ .h^j
- Ll
^\
1- r y
[j".
:U
■I I
-I I
I t
,,>)p,
4. ihiiii I
X
. i
f V
4
■ f
" ;
I. . ■ ,
■y;
5*
U' ^^'
■■v;
I I
;^
^
•i^
tai
iir
1 . ■■ J
64
the former) and at the diflance, perhaps, of ten or
fifteen leagues from the coaft.
For,
95. Firft^ The dirediion, in which the earthquake
arrived at Liibon, was from the north-weft ; at Ma-
deira it came from the north-eaft^ and in England
It came from the fouth-Weft ; all of which perfedly
agree with the place affumed
g6. Secondly^ The times in which the earthquake
arrived at different places, agree perfedly well alfp
*
with the fame point.
And,
compute
degree of a great circle from Lifbon, and
^j. Thirdly, The interval between thefe, and the
time of the arrival of the fiibfequent wave, concur in
confirming it. That all this might appear the better,
I have fubjoined the following table,- alTuming the
point, from whence I compute, at the diftance of
about a
a degree and half from Oporto. In confequence of
this fuppofition, I have added three minutes to the
interval betvveen the time when the fhock was felt
at Lifbon, and at the feveral other places. The firft
column in the table contains the names of places ;
the fecond, the diftances from the afTunied point,
reckoned in half degrees; the third, the time that
the earthquake took up in travelling to each, ex-
prefled in minutes 3 and the fourth contains the time
in which the wave was propagated,, from its fource to
the refpedtive places, exprefled in minutes likewife.
£
quake,
re<3:ions
different places.
(two or three only excepted ) are taken from the 49th volume of the
Tranf. and the
To
muft
very numerous, I was not willing to quote at Ien|
they
Lifbon
\
i
A
'" .. 1; •
(
LIfbon *
Oporto *
" Ayamonte
Cadiz -
Madrid -
Gibraltar ■
Madeira -
Mountfbay
Plymouth
Portfmouth
Kingfale
Swanfea -
The Hague
Lochnefs ■
Antigua -
M^
Halfdeg.j MIn.
2
3
6
9
9
XI
20
21
24
33
98
Barbadoes - - - - - 1 loi
3
5
12
II
18
2i
ap
32
66
^
Min.
r
■1
12
53
82
152
267
60
5
290
i3<^
5^5
485
98. In computing the times in the preceding table
allowance was made for the difference of longitude
as it is laid down in the common maps, which an
- 1
,*
L"
t
I
i.
1 !:
\
4
r
l\l\
I
f
■ X
-T
t
\
k
h*-^
o
I
* It appears, by all the accourits, that the interval between the
earthquake and wave, either at Oporto or Lifbon, was not long:
I have met with no account yet, however, which tells us how long
it was at the former, and only one which mentions it at the latter,
where it is faid to have been nine minutes. [See Memoires fur ies
315,] -Thefe
P
ierved, perhaps, to afcertain the diftance of thofe two places from
the original fource a little more accurately ; but, as the diftance of
neither from thence could be very great, a fmaH difference in them
would hardly fenfibly affed any of the others ; from which, there-
fore, v^^e may draw the fame general conclufions, a^ if they were
cxad:/
I
not
■^
i-r ^ —
1^
■ —J.
\f
i
'IS
-ti
^1
■C
I
t
! f
I \
I t!
Ill** I
l^
II
i -I
^^.
i \^'-^ ■• '•ii
'^s
'■i^^'t
J
I
'I
' I
* 1
J
m
■i;
^ F
:f
r.^*'' '
- \
h- -
r
h
I I'
mi
:MS
' h
\mm
\
■T
'*i.'
I
mi^\
■,t
r1
; \
\
I I
/»'l'*-'l(i»*
66
always greatly to be depended
\
I*
on. The times
themfelves alfo are often fo carelelly obferved, as
well as vaguely related, that they are many of them
fubjea
qnfiderable errors : the concurrent tefti
monies, however, are fo many, that there can be
dciibt about the main point -, and, that the errors
might be as fmail
offible, I have not only
^eavoured to feled: thole accounts that had the greateft
appearance of accuracy, but, in all cafes where it was
to be had, I have always taken a mean amongft them.
In many of the accounts, the relaters fay only be-
tween fuch hours
about fuch
hour
this
kind were the accounts of the times of the agitation
of the waters at The Hague and Lochnefs, which
vary the moft from a medium of the reft, the former
g about k\Qn minutes in defed:, and the latter
regard to
, from the
about twenty minutes in~ excefs ^ with
the latter, however, I muft -obferve, that
aqcount itfelf, it is probable the agitation happened
fooner than eleven o'clock, which is the time men-
•^ -
tioned. The accounts alfo of the time of the agita
tlon of the waters in the northern parts of England
{QQvn to confirm the fame thing *.
9p. It is obfervable, in the preceding table,
the times, which the wave took up in travelling
4 4
w
^ As the fhorteft way that the vapoyr could pafs from near Lifbon
to Lochnefs was under the ocean.
it might, on that ac-
count, be fomewhat retarded ; for the water adding to the weight
of the fuperincumbent mafs, and not to its elafticity, muft produce
this efFe<^ in fome degree; it is probable, however, that this could
make no great difference, as the motion feems to have been very
little retarded in its paflagc from the original fource to Madeira, to
which place, I fiippofe, it muft have paffed under deeper feas tha»
would be found in its road to Scotland^
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the fame propQFtion With th'e dif^ances of th^
fpedive
from thev fuppofed fource of the
motion
th
howeVen is
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e ^^/
point alfumed, fnice it is manifeft, wherever it was
k
that it could not be far from Liibo
well becaufe
the wave arrived there fo vefy foon after the earth-
as becaufe it was fo great, rifing, as we are
the diftance of three miles from Lifbon, to
The true reafon of
the height of fifty or fixty feet
this difpropdrtion, feem
be the difference
the
depth of the water ; for, in every inft^nce in the abovQ
table, the time will be found to be proportionably
fliorter or longer, as the water through which the
wave pafTed was * deeper or fhallower. Thus the
motion of the wave to Kingfale or Mountfbay (through
waters not deeper in general than aoo fathoms) was
flower than that to Madeira, (where the waters are
much deeper) in the proportion of about three to
iivev and it was flower than that to Barbadoes,
(where its eourfe lay tlirough the deepeft part of the
Atlantic
ly in th6 proportion of
to
three: fo likewife the motion of it from the Scilly
iflands to Swanfea in Wales (where the depth gra-
dually dimimfliesfrorii about fixty or • feventy fa-
thoms to a very fmall matter) was ilill flower than
that to Kingfale, in the proportion of lefs thaft
to three:: the fam€ th
obfervable.with regard
iliiiiiiiift
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We
in
deep waters, move with a velocity that would carry them round
the whole earth in a fmgle day ; but as they get mto " _
waters, they are greatly retarded : and -we are told, that in the
river of Amazons, the fame tide is found running up to the tenth
or twelfth day, before it is entirely fpent. [See Condaminis Voyage^
the Maranon.'X
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Plymouth
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68
Plymouth alfo. where the wave arrived about ninety
minutes later than at Mountfbay, though the difFerence
ot their diftance from the iirft fource could not, upon
any fuppofition, be more than forty or fifty miles.
't.
Sect. VIIJ.
100. If we would inquire into the depth, at which
the caufe lies, that occafions any particular earth-
quake, I know of no method of determining it, which
does not require obfervations not yet to be hadj but
if fuch could be procured, and they were made with
fufficient accuracy, I think fome kind of guefs might
be formed concerning it : "
,./°^* ^^^' ^" *^^^e inftances, where the vapour
difcharges itfelf at the mouths of volcanos, (
for
cafe of the earthquake at Lima)
might, perhap
be poffible for a careful obferver to trace th« * thick-
nefs of the feveral flrata from thence to the place
where the earthquake took its rife, or at leafl as far
took its rife from under the fea^
the (hore. if
If this could be once done
any one inftance, and
th& velocity of fuch an earthquake nicely determined
we might then guefs at the depth of the caufe
y
Gt|ier earthquakes, where we knew their velocity, by
taking the f depths proportional to thofe velocities
which probably would anfwer very nearly. *
10%. Secondly] If, in any inftance, it fhould be
poffible to know how much the motion of any earth-
quake was retarded by paffing under the ocean, tho
Th
ing up the hills, come toil
See art. 43. and Fig. 3,
+ See the note to art. 6^
manner
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69
depth of the ocean being known, the depth at which,
the vapour paiTed would be known alfo 5 for the ve-
locity under the water would be to the velocity, if
there had been no water, in the fubduplicate ratio of
the weight in the latter cafe to the weight in the
former j hence allowing earth to be about two and
half times the weight of. water, the depth will be
readily found. .
103. Thirdly, Let us conceiye the earth to be
formed according to the idea before given of it, and
that the fame flrata are at a medium of the fame
thicknefs for a very great extent, as well in thofe
places, where feveral of the upper ones are wanting,
as where they are not. Upon this fuppofitlon, we
may difcover the depthj at which the' vapour palTes,
by comparing the feveral velocities of the fame earth-'
quake in places, where the * thipkneffes of the fuper-
incumbent mafs are different. ■ It rniill be acknow-
legcd, indeed, that fuch obfervations with regard ta
time, as would enable us to determine thefe veloci-
ties, are in general much too nice to be expedied :
the matter, however, is not altogether defperate, as
we may colle(5l them, in fome meafure perhaps, from
other circumftances, fuch, for inftance, as the degree
of -]• agitation in different waters, the proportional
J fuddennefs, with which the earth is lifted in dif-
ferent places, ^c,
104. As
■v
* In order to know this difference, it will be neceflary to trace
the thicknefs of thofe ftrata, which are found in fome of the places^
but are wanting m others.
• f See art. 71 and 72.
X This may be kno^n from the diftance to which the mercury
fubfides in the barometer, upon the firft railing of the earth by the
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70
104. As the obfervations relating to the earthquake
of the I ft of Noyember 1755 are too grofs, it would
be in vain to attempt, by any of the foregoing me
thods, to detejrmine with any certainty the depth at
which the eaule of it lay'; but, if I might be allowed
to form a randori guefs about it, I.fhould fuppofe.
(upon a comparifon of all circumftances) that it could
not be much lefs than a mile, or a mile and half, and
I think it is probable, it did not exceed three, miles.
C O N C L U S ION.
IOC. Thus have I endeavoured to fhew how the,
principal phgenomena of earthquakes may be pro-
duced, by a eaufq with which none, that I ha ve feen , ^
appear to me. to be incompatible. As J h^ve not
knowingly mifrepirefented any fadt, fo neither have I
defignedly omitted any that appeared to affect the main
queftion; but, that I might not unneceflarily fwell
what had already much exceeded the limits at iirft in-
tended for it, I have omitted,.
1 06.
Thofe minuter
which
almoft every reader would eaiily account for, from
what. has been faid already, and which did not feem
to lead to any thing farther: fuch, for inftaiice,; are
the fudden flopping and gufhing out of fountains, oc-
calloned by the opening or contrading of .fiflures^ tha
dizzinefs and ificknefs people feel, from the almoft
imperceptible wave-like motion, &c.
■^-4
vapour
:hquake
omenon, which is a cpnimon
I of November J755> except at Amfter-
dam, where the mercury fubfided more than an inch.; See Jiift#
Philof. of Earthq. p, 309
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107. Secondfyy Thofe appearances which feemed
to depend upon particular circumftances, and of
which, therefore, unlefs we had a more exad: know-
lege of the countries where they happened, it would
have been impoffiblc to give any account, without
having recourfe to uncertain conjectures j of this kind,
was the greater agitation of the waters in the lakes of
Switzerland, at the time of the earthquake of the ift
of November 1 7 5" f, than during the * earthquake of
the 9th of December following, though the houfes
upon the borders of them were more violently fhaken
by the latter. And,
108. Z^y?/^, Thofe appearances, which only feem
to have an accidental connedion with earthquakes, or
the caufes of them 3 of this kind, are the effeds which,
in fome inftances perhaps, they produce on the wea-
ther J the diftempers which are fometimes faid to fuc-
ceedthem; the difturbance which, we are told, they
the
■-,
have fometimes occafioned, during the ihocks
direction of the magnetic needle, ^c, none of which
are obferved to be conftant attendants on earthquakes,
nor do they feem materially to affed the folution given
either one way or other.
^
* See Monfieur Bertrand's Mmoires fur Us trmiUmm d$
Terre,
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